Aerosol method and apparatus, particulate products, and electronic devices made therefrom

ABSTRACT

Provided is an aerosol method, and accompanying apparatus, for preparing powdered products of a variety of materials involving the use of an ultrasonic aerosol generator ( 106 ) including a plurality of ultrasonic transducers ( 120 ) underlying and ultrasonically energizing a reservoir of liquid feed ( 102 ) which forms droplets of the aerosol. Carrier gas ( 104 ) is delivered to different portions of the reservoir by a plurality of gas delivery ports ( 136 ) delivering gas from a gas delivery system. The aerosol is pyrolyzed to form particles, which are then cooled and collected. The invention also provides powders made by the method and devices made using the powders.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/668,947 filed Sep. 22, 2000, now U.S. Pat. No. 6,635,348, which is adivisional application of U.S. patent application Ser. No. 09/030,057filed Feb. 24, 1998 now U.S. Pat. No. 6,338,809, which claims priorityto U.S. Provisional Patent Application No. 60/039,450 filed Feb. 24,1997 and to U.S. Provisional Patent Application No. 60/038,258 filedFeb. 24, 1997, the contents of all of which are incorporated herein asif set forth herein in full.

STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH/DEVELOPMENT

This invention was made with Government support under contractsN00014-95-C-0278 and N00014-96-C-0395 awarded by the Office of NavalResearch. The Government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention involves aerosol production of finely-dividedparticles of a variety of compositions. The present invention alsoinvolves the particles so manufactured and electronic devices made usingthe particles.

BACKGROUND OF THE INVENTION

Powdered materials are used in many manufacturing processes. One largeuse for powders is for thick film deposition to prepare films of avariety of materials. Some thick film applications include, for example,deposition of phosphor materials for flat panel displays, and patterningof eclectically conductive features for electronic products.

For thick film applications, and for other applications, there is atrend to use powders of ever smaller particles. Generally desirablefeatures in small particles include a small particle size; a narrowparticle size distribution; a dense, spherical particle morphology; anda crystalline grain structure. Existing technologies for preparingpowdered products, however, often could be improved with respect toattaining all, or substantially all, of these desired features forparticles used in thick film applications.

One method that has been used to make small particles is to precipitatethe particles from a liquid medium. Such liquid precipitation techniquesare often difficult to control to produce particles with the desiredcharacteristics. Also, particles prepared by liquid precipitation routesoften are contaminated with significant quantities of surfactants orother organic materials used during the liquid phase processing.

Aerosol methods have also been used to make a variety of smallparticles. One aerosol method for making small particles is spraypyrolysis, in which an aerosol spray is generated and then converted ina reactor to the desired particles. Spray pyrolysis systems have,however, been mostly experimental, and unsuitable for commercialparticle production. Furthermore, control of particle size distributionis a concern with spray pyrolysis. Also, spray pyrolysis systems areoften inefficient in the use of carrier gases that suspend and carryliquid droplets of the aerosol. This inefficiency is a majorconsideration for commercial applications of spray pyrolysis systems.

There is a significant need for improved manufacture techniques formaking powders of small particles for use in thick film and otherapplications.

Not only would improved particle manufacture techniques be desirable,but improved materials would also be desirable for a variety ofapplications. For example, there is a significant problem in cofireprocesses, such as cofiring of multi-layer ceramic capacitors and othercomponents, of delaminations and other failures that can occur due tosintering/densification/shrinkage mismatch between adjoining layers.Improved techniques for providing high quality particles to reduce theseproblems would be desirable.

SUMMARY OF THE INVENTION

The present invention provides an aerosol process for manufacturingfinely-divided powders of a variety of materials having desirableproperties and at commercially acceptable rates. Apparatus is alsoprovided for implementing the manufacturing method.

An important aspect of the present invention is aerosol generation. Anaerosol generator and aerosol generation method are provided that arecapable of producing large quantities of a high quality, dense aerosolfor spray pyrolysis operations. This is significantly different fromaerosol generation that has previously occurred with respect to spraypyrolysis particle manufacture in small-scale, laboratory systems. Anaerosol generator is provided including an array of ultrasonictransducers underlying a single reservoir of precursor solution that isultrasonically energized to produce the aerosol. Careful distribution ofcarrier gas to different portions of the reservoir results in anefficient use of carrier gas in making a dense aerosol and at a highrate suitable for commercial applications.

The process is versatile for preparing powders of a number of materials.An important group of powders prepared with the process of the presentinvention include multi-phase particles.

Particularly advantageous are multi-phase particles designed for use inmanufacturing electrically-conductive metallic films for electronicproducts. The multi-phase particles include a metallic phase and anon-metallic phase. In one preferred type of multi-phase particles, thenon-metallic phase comprises at least one of silica, alumina, titaniaand zirconia. In another preferred type of multi-phase particles, thenon-metallic phase includes a titanate, such as barium titanate,neodymium titanate or other titanates as discussed below.

Yet another important type of multi-phase particles of the presentinvention include those having a metallic phase and a non-metallic phaseincluding elemental carbon. These multi-phase particles are useful aselectrode materials and as catalysts.

The present invention also provides electronic products including adielectric layer adjoining an electrically conductive film that has beenformed using multi-phase particles of the present invention, andespecially using multi-phase particles including a titanate as thenon-metallic phase. In this way, the electrically conductive film may becofired with a titanate dielectric layer with improved compatibilitybetween the layers, for reduced delaminations and other failures.

These and other aspects of the invention are discussed in more detailbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process block diagram showing one embodiment of the processof the present invention.

FIG. 2 is a side view of a furnace and showing one embodiment of thepresent invention for sealing the end of a furnace tube.

FIG. 3 is a view of the side of an end cap that faces away from thefurnace shown in FIG. 2.

FIG. 4 is a view of the side of an end cap that faces toward the furnaceshown in FIG. 2.

FIG. 5 is a side view in cross section of one embodiment of aerosolgenerator of the present invention.

FIG. 6 is a top view of a transducer mounting plate showing a 49transducer array for use in an aerosol generator of the presentinvention.

FIG. 7 is a top view of a transducer mounting plate for a 400 transducerarray for use in an ultrasonic generator of the present invention.

FIG. 8 is a side view of the transducer mounting plate shown in FIG. 7.

FIG. 9 is a partial side view showing the profile of a single transducermounting receptacle of the transducer mounting plate shown in FIG. 7.

FIG. 10 is a partial side view in cross-section showing an alternativeembodiment for mounting an ultrasonic transducer.

FIG. 11 is a top view of a bottom retaining plate for retaining aseparator for use in an aerosol generator of the present invention.

FIG. 12 is a top view of a liquid feed box having a bottom retainingplate to assist in retaining a separator for use in an aerosol generatorof the present invention.

FIG. 13 is a side view of the liquid feed box shown in FIG. 8.

FIG. 14 is a side view of a gas tube for delivering gas within anaerosol generator of the present invention.

FIG. 15 shows a partial top view of gas tubes positioned in a liquidfeed box for distributing gas relative to ultrasonic transducerpositions for use in an aerosol generator of the present invention.

FIG. 16 shows one embodiment for a gas distribution configuration forthe aerosol generator of the present invention.

FIG. 17 shows another embodiment for a gas distribution configurationfor the aerosol generator of the present invention.

FIG. 18 is a top view of one embodiment of a gas distribution plate/gastube assembly of the aerosol generator of the present invention.

FIG. 19 is a side view of one embodiment of the gas distributionplate/gas tube assembly shown in FIG. 18.

FIG. 20 shows one embodiment for orienting a transducer in the aerosolgenerator of the present invention.

FIG. 21 is a top view of a gas manifold for distributing gas within anaerosol generator of the present invention.

FIG. 22 is a side view of the gas manifold shown in FIG. 21.

FIG. 23 is a top view of a generator lid of a hood design for use in anaerosol generator of the present invention.

FIG. 24 is a side view of the generator lid shown in FIG. 23.

FIG. 25 is a process block diagram of one embodiment in the presentinvention including an aerosol concentrator.

FIG. 26 is a top view in cross section of a virtual impactor that may beused for concentrating an aerosol according to the present invention.

FIG. 27 is a front view of an upstream plate assembly of the virtualimpactor shown in FIG. 26.

FIG. 28 is a top view of the upstream plate assembly shown in FIG. 27.

FIG. 29 is a side view of the upstream plate assembly shown in FIG. 27.

FIG. 30 is a front view of a downstream plate assembly of the virtualimpactor shown in FIG. 26.

FIG. 31 is a top view of the downstream plate assembly shown in FIG. 30.

FIG. 32 is a side view of the downstream plate assembly shown in FIG.30.

FIG. 33 is a process block diagram of one embodiment of the process ofthe present invention including a droplet classifier.

FIG. 34 is a top view in cross section of an impactor of the presentinvention for use in classifying an aerosol.

FIG. 35 is a front view of a flow control plate of the impactor shown inFIG. 34.

FIG. 36 is a front view of a mounting plate of the impactor shown inFIG. 34.

FIG. 37 is a front view of an impactor plate assembly of the impactorshown in FIG. 34.

FIG. 38 is a side view of the impactor plate assembly shown in FIG. 37.

FIG. 39 shows a side view in cross section of a virtual impactor incombination with an impactor of the present invention for concentratingand classifying droplets in an aerosol.

FIG. 40 is a process block diagram of one embodiment of the presentinvention including a particle cooler.

FIG. 41 is a top view of a gas quench cooler of the present invention.

FIG. 42 is an end view of the gas quench cooler shown in FIG. 41.

FIG. 43 is a side view of a perforated conduit of the quench coolershown in FIG. 41.

FIG. 44 is a side view showing one embodiment of a gas quench cooler ofthe present invention connected with a cyclone.

FIG. 45 is a process block diagram of one embodiment of the presentinvention including a particle coater.

FIG. 46 is a block diagram of one embodiment of the present inventionincluding a particle modifier.

FIG. 47 shows cross sections of various particle morphologies of somecomposite particles manufacturable according to the present invention.

FIG. 48 shows a side view of one embodiment of apparatus of the presentinvention including an aerosol generator, an aerosol concentrator, adroplet classifier, a furnace, a particle cooler, and a particlecollector.

FIG. 49 is a block diagram of one embodiment of the process of thepresent invention including the addition of a dry gas between theaerosol generator and the furnace.

DETAILED DESCRIPTION OF THE INVENTION

In one aspect, the present invention provides a method for preparing aparticulate product. A feed of liquid-containing, flowable medium,including at least one precursor for the desired particulate product, isconverted to aerosol form, with droplets of the medium being dispersedin and suspended by a carrier gas. Liquid from the droplets in theaerosol is then removed to permit formation in a dispersed state of thedesired particles. Typically, the feed precursor is pyrolyzed in afurnace to make the particles. In one embodiment, the particles aresubjected, while still in a dispersed state, to compositional orstructural modification, if desired. Compositional modification mayinclude, for example, coating the particles. Structural modification mayinclude, for example, crystallization, recrystallization ormorphological alteration of the particles. The term powder is often usedherein to refer to the particulate product of the present invention. Theuse of the term powder does not indicate, however, that the particulateproduct must be dry or in any particular environment. Although theparticulate product is typically manufactured in a dry state, theparticulate product may, after manufacture, be placed in a wetenvironment, such as in a slurry.

The process of the present invention is particularly well suited for theproduction of particulate products of finely divided particles having aweight average size, for most applications, in a range having a lowerlimit of about 0.1 micron, preferably about 0.3 micron, more preferablyabout 0.5 micron and most preferably about 0.8 micron; and having anupper limit of about 4 microns, preferably about 3 microns, morepreferably about 2.5 microns and more preferably about 2 microns. Aparticularly preferred range for many applications is a weight averagesize of from about 0.5 micron to about 3 microns, and more particularlyfrom about 0.5 micron to about 2 microns. For some applications,however, other weight average particle sizes may be particularlypreferred.

In addition to making particles within a desired range of weight averageparticle size, with the present invention the particles may be producedwith a desirably narrow size distribution, thereby providing sizeuniformity that is desired for many applications.

In addition to control over particle size and size distribution, themethod of the present invention provides significant flexibility forproducing particles of varying composition, crystallinity andmorphology. For example, the present invention may be used to producehomogeneous particles involving only a single phase or multi-phaseparticles including multiple phases. In the case of multi-phaseparticles, the phases may be present in a variety of morphologies. Forexample, one phase may be uniformly dispersed throughout a matrix ofanother phase. Alternatively, one phase may form an interior core whileanother phase forms a coating that surrounds the core. Othermorphologies are also possible, as discussed more fully below.

Referring now to FIG. 1, one embodiment of the process of the presentinvention is described. A liquid feed 102, including at least oneprecursor for the desired particles, and a carrier gas 104 are fed to anaerosol generator 106 where an aerosol 108 is produced. The aerosol 108is then fed to a furnace 110 where liquid in the aerosol 108 is removedto produce particles 112 that are dispersed in and suspended by gasexiting the furnace 110. The particles 112 are then collected in aparticle collector 114 to produce a particulate product 116.

As used herein, the liquid feed 102 is a feed that includes one or moreflowable liquids as the major constituent(s), such that the feed is aflowable medium. The liquid feed 102 need not comprise only liquidconstituents. The liquid feed 102 may comprise only constituents in oneor more liquid phase, or it may also include particulate materialsuspended in a liquid phase. The liquid feed 102 must, however, becapable of being atomized to form droplets of sufficiently small sizefor preparation of the aerosol 108. Therefore, if the liquid feed 102includes suspended particles, those particles should be relatively smallin relation to the size of droplets in the aerosol 108. Such suspendedparticles should typically be smaller than about 1 micron in size,preferably smaller than about 0.5 micron in size, and more preferablysmaller than about 0.3 micron in size and most preferably smaller thanabout 0.1 micron in size. Most preferably, the suspended particlesshould be able to form a colloid. The suspended particles could befinely divided particles, or could be agglomerate masses comprised ofagglomerated smaller primary particles. For example, 0.5 micronparticles could be agglomerates of nanometer-sized primary particles.When the liquid feed 102 includes suspended particles, the particlestypically comprise no greater than about 25 to 50 weight percent of theliquid feed.

As noted, the liquid feed 102 includes at least one precursor forpreparation of the particles 112. The precursor may be a substance ineither a liquid or solid phase of the liquid feed 102. Frequently, theprecursor will be a material, such as a salt, dissolved in a liquidsolvent of the liquid feed 102. Typical precursor salts include nitrate,chloride, sulfate, acetate and oxalate salts and the like. The precursormay undergo one or more chemical reactions in the furnace 110 to assistin production of the particles 112. Alternatively, the precursormaterial may contribute to formation of the particles 112 withoutundergoing chemical reaction. This could be the case, for example, whenthe liquid feed 102 includes, as a precursor material, suspendedparticles that are not chemically modified in the furnace 110. In anyevent, the particles 112 comprise at least one component originallycontributed by the precursor.

The liquid feed 102 may include multiple precursor materials, which maybe present together in a single phase or separately in multiple phases.For example, the liquid feed 102 may include multiple precursors insolution in a single liquid vehicle. Alternatively, one precursormaterial could be in a solid particulate phase and a second precursormaterial could be in a liquid phase. Also, one precursor material couldbe in one liquid phase and a second precursor material could be in asecond liquid phase, such as could be the case when the liquid feed 102comprises an emulsion. Different components contributed by differentprecursors may be present in the particles together in a single materialphase, or the different components may be present in different materialphases when the particles 112 are composites of multiple phases.

When the liquid feed 102 includes a soluble precursor, the precursorsolution should be unsaturated to avoid the formation of precipitates.Solutions of salts will typically be used in concentrations in a rangeto provide a solution including from about 1 to about 50 weight percentsolute. Most often, the liquid feed will include a solution with fromabout 5 weight percent to about 40 weight percent solute, and morepreferably to about 30 weight percent solute. Preferably the solvent isaqueous-based for ease of operation, although other solvents, such astoluene or other organic solvents, may be desirable for specificmaterials. The use of organic solvents, however, can sometimes lead toundesirable carbon contamination in the particles. The pH of theaqueous-based solutions can be adjusted to alter the solubilitycharacteristics of the precursor or precursors in the solution.

The carrier gas 104 may comprise any gaseous medium in which dropletsproduced from the liquid feed 102 may be dispersed in aerosol form.Also, the carrier gas 104 may be inert, in that the carrier gas 104 doesnot participate in formation of the particles 112. Alternatively, thecarrier gas may have one or more active component(s) that contribute toformation of the particles 112. In that regard, the carrier gas mayinclude one or more reactive components that react in the furnace 110 tocontribute to formation of the particles 112.

The aerosol generator 106 atomizes the liquid feed 102 to form dropletsin a manner to permit the carrier gas 104 to sweep the droplets away toform the aerosol 108. The droplets comprise liquid from the liquid feed102. The droplets may, however, also include nonliquid material, such asone or more small particles held in the droplet by the liquid. Forexample, when the particles 112 are composite, or multi-phase,particles, one phase of the composite may be provided in the liquid feed102 in the form of suspended precursor particles and a second phase ofthe composite may be produced in the furnace 110 from one or moreprecursors in the liquid phase of the liquid feed 102. Furthermore theprecursor particles could be included in the liquid feed 102, andtherefore also in droplets of the aerosol 108, for the purpose only ofdispersing the particles for subsequent compositional or structuralmodification during or after processing in the furnace 110.

An important aspect of the present invention is generation of theaerosol 108 with droplets of a small average size, narrow sizedistribution. In this manner, the particles 112 may be produced at adesired small size with a narrow size distribution, which areadvantageous for many applications.

The aerosol generator 106 is capable of producing the aerosol 108 suchthat it includes droplets having a weight average size in a range havinga lower limit of about 1 micron and preferably about 2 microns; and anupper limit of about 10 microns, preferably about 7 microns, morepreferably about 5 microns and most preferably about 4 microns. A weightaverage droplet size in a range of from about 2 microns to about 4microns is more preferred for most applications, with a weight averagedroplet size of about 3 microns being particularly preferred for someapplications. The aerosol generator is also capable of producing theaerosol 108 such that it includes droplets in a narrow sizedistribution. Preferably, the droplets in the aerosol are such that atleast about 70 percent (more preferably at least about 80 weight percentand most preferably at least about 85 weight percent) of the dropletsare smaller than about 10 microns and more preferably at least about 70weight percent (more preferably at least about 80 weight percent andmost preferably at least about 85 weight percent) are smaller than about5 microns. Furthermore, preferably no greater than about 30 weightpercent, more preferably no greater than about 25 weight percent andmost preferably no greater than about 20 weight percent, of the dropletsin the aerosol 108 are larger than about twice the weight averagedroplet size.

Another important aspect of the present invention is that the aerosol108 may be generated without consuming excessive amounts of the carriergas 104. The aerosol generator 106 is capable of producing the aerosol108 such that it has a high loading, or high concentration, of theliquid feed 102 in droplet form. In that regard, the aerosol 108preferably includes greater than about 1×10⁶ droplets per cubiccentimeter of the aerosol 108, more preferably greater than about 5×10⁶droplets per cubic centimeter, still more preferably greater than about1×10⁷ droplets per cubic centimeter, and most preferably greater thanabout 5×10⁷ droplets per cubic centimeter. That the aerosol generator106 can produce such a heavily loaded aerosol 108 is particularlysurprising considering the high quality of the aerosol 108 with respectto small average droplet size and narrow droplet size distribution.Typically, droplet loading in the aerosol is such that the volumetricratio of liquid feed 102 to carrier gas 104 in the aerosol 108 is largerthan about 0.04 milliliters of liquid feed 102 per liter of carrier gas104 in the aerosol 108, preferably larger than about 0.083 millilitersof liquid feed 102 per liter of carrier gas 104 in the aerosol 108, morepreferably larger than about 0.167 milliliters of liquid feed 102 perliter of carrier gas 104, still more preferably larger than about 0.25milliliters of liquid feed 102 per liter of carrier gas 104, and mostpreferably larger than about 0.333 milliliters of liquid feed 102 perliter of carrier gas 104.

This capability of the aerosol generator 106 to produce a heavily loadedaerosol 108 is even more surprising given the high droplet output rateof which the aerosol generator 106 is capable, as discussed more fullybelow. It will be appreciated that the concentration of liquid feed 102in the aerosol 108 will depend upon the specific components andattributes of the liquid feed 102 and, particularly, the size of thedroplets in the aerosol 108. For example, when the average droplet sizeis from about 2 microns to about 4 microns, the droplet loading ispreferably larger than about 0.15 milliliters of aerosol feed 102 perliter of carrier gas 104, more preferably larger than about 0.2milliliters of liquid feed 102 per liter of carrier gas 104, even morepreferably larger than about 0.2 milliliters of liquid feed 102 perliter of carrier gas 104, and most preferably larger than about 0.3milliliters of liquid feed 102 per liter of carrier gas 104. Whenreference is made herein to liters of carrier gas 104, it refers to thevolume that the carrier gas 104 would occupy under conditions ofstandard temperature and pressure.

The furnace 110 may be any suitable device for heating the aerosol 108to evaporate liquid from the droplets of the aerosol 108 and therebypermit formation of the particles 112. For most applications, maximumaverage stream temperatures in the furnace 110 will generally be in arange of from about 500° C. to about 1500° C., and preferably in therange of from about 900° C. to about 1300° C. The maximum average streamtemperature refers to the maximum average temperature that an aerosolstream attains while flowing through the furnace. This is typicallydetermined by a temperature probe inserted into the furnace.

Although longer residence times are possible, for many applications,residence time in the heating zone of the furnace 110 of shorter thanabout 4 seconds is typical, with shorter than about 2 seconds beingpreferred, shorter than about 1 second being more preferred, shorterthan about 0.5 second being even more preferred, and shorter than about0.2 second being most preferred. The residence time should be longenough, however, to assure that the particles 112 attain the desiredmaximum average stream temperature for a given heat transfer rate. Inthat regard, with extremely short residence times, higher furnacetemperatures could be used to increase the rate of heat transfer so longas the particles 112 attain a maximum temperature within the desiredstream temperature range. That mode of operation, however, is notpreferred. Also, it is preferred that, in most cases, the maximumaverage stream temperature not be attained in the furnace 110 untilsubstantially at the end of the heating zone in the furnace 110. Forexample, the heating zone will often include a plurality of heatingsections that are each independently controllable. The maximum averagestream temperature should typically not be attained until the finalheating section, and more preferably until substantially at the end ofthe last heating section. This is important to reduce the potential forthermophoretic losses of material. Also, it is noted that as usedherein, residence time refers to the actual time for a material to passthrough the relevant process equipment. In the case of the furnace, thisincludes the effect of increasing velocity with gas expansion due toheating.

Typically, the furnace 110 will be a tube-shaped furnace, so that theaerosol 108 moving into and through the furnace does not encounter sharpedges on which droplets could collect. Loss of droplets to collection atsharp surfaces results in a lower yield of particles 112. Moreimportant, however, the accumulation of liquid at sharp edges can resultin re-release of undesirably large droplets back into the aerosol 108,which can cause contamination of the particulate product 116 withundesirably large particles. Also, over time, such liquid collection atsharp surfaces can cause fouling of process equipment, impairing processperformance.

The furnace 110 may include a heating tube made of any suitablematerial. The tube material may be a ceramic material, for example,mullite, silica or alumina. Alternatively, the tube may be metallic.Advantages of using a metallic tube are low cost, ability to withstandsteep temperature gradients and large thermal shocks, machinability andweldability, and ease of providing a seal between the tube and otherprocess equipment. Disadvantages of using a metallic tube includelimited operating temperature and increased reactivity in some reactionsystems.

When a metallic tube is used in the furnace 110, it is preferably a highnickel content stainless steel alloy, such as a 330 stainless steel, ora nickel-based super alloy. As noted, one of the major advantages ofusing a metallic tube is that the tube is relatively easy to seal withother process equipment. In that regard, flange fittings may be weldeddirectly to the tube for connecting with other process equipment.Metallic tubes are generally preferred for making particles that do notrequire a maximum tube wall temperature of higher than about 1100° C.during particle manufacture.

When higher temperatures are required, ceramic tubes are typically used.One major problem with ceramic tubes, however, is that the tubes can bedifficult to seal with other process equipment, especially when the endsof the tubes are maintained at relatively high temperatures, as is oftenthe case with the present invention.

One configuration for sealing a ceramic tube is shown in FIGS. 2, 3 and4. The furnace 110 includes a ceramic tube 374 having an end cap 376fitted to each end of the tube 374, with a gasket 378 disposed betweencorresponding ends of the ceramic tube 374 and the end caps 376. Thegasket may be of any suitable material for sealing at the temperatureencountered at the ends of the tubes 374. Examples of gasket materialsfor sealing at temperatures below about 250° C. include silicone,TEFLON™ and VITON™. Examples of gasket materials for higher temperaturesinclude graphite, ceramic paper, thin sheet metal, and combinationsthereof.

Tension rods 380 extend over the length of the furnace 110 and throughrod holes 382 through the end caps 376. The tension rods 380 are held intension by the force of springs 384 bearing against bearing plates 386and the end caps 376. The tube 374 is, therefore, in compression due tothe force of the springs 384. The springs 384 may be compressed anydesired amount to form a seal between the end caps 376 and the ceramictube 374 through the gasket 378. As will be appreciated, by using thesprings 384, the tube 374 is free to move to some degree as it expandsupon heating and contracts upon cooling. To form the seal between theend caps 376 and the ceramic tube 374, one of the gaskets 378 is seatedin a gasket seat 388 on the side of each end cap 376 facing the tube374. A mating face 390 on the side of each of the end caps 376 facesaway from the tube 374, for mating with a flange surface for connectionwith an adjacent piece of equipment.

Also, although the present invention is described with primary referenceto a furnace reactor, which is preferred, it should be recognized that,except as noted, any other thermal reactor, including a flame reactor ora plasma reactor, could be used instead. A furnace reactor is, however,preferred, because of the generally even heating characteristic of afurnace for attaining a uniform stream temperature.

The particle collector 114, may be any suitable apparatus for collectingparticles 112 to produce the particulate product 116. One preferredembodiment of the particle collector 114 uses one or more filter toseparate the particles 112 from gas. Such a filter may be of any type,including a bag filter. Another preferred embodiment of the particlecollector uses one or more cyclone to separate the particles 112. Otherapparatus that may be used in the particle collector 114 includes anelectrostatic precipitator. Also, collection should normally occur at atemperature above the condensation temperature of the gas stream inwhich the particles 112 are suspended. Also, collection should normallybe at a temperature that is low enough to prevent significantagglomeration of the particles 112.

The process and apparatus of the present invention are well-suited forproducing commercial-size batches of extremely high quality particles.In that regard, the process and the accompanying apparatus provideversatility for preparing powder including a wide variety of materials,and easily accommodate shifting of production between differentspecialty batches of particles.

Of significant importance to the operation of the process of the presentinvention is the aerosol generator 106, which must be capable ofproducing a high quality aerosol with high droplet loading, aspreviously noted. With reference to FIG. 5, one embodiment of an aerosolgenerator 106 of the present invention is described. The aerosolgenerator 106 includes a plurality of ultrasonic transducer discs 120that are each mounted in a transducer housing 122. The transducerhousings 122 are mounted to a transducer mounting plate 124, creating anarray of the ultrasonic transducer discs 120. Any convenient spacing maybe used for the ultrasonic transducer discs 120. Center-to-centerspacing of the ultrasonic transducer discs 120 of about 4 centimeters isoften adequate. The aerosol generator 106, as shown in FIG. 5, includesforty-nine transducers in a 7×7 array. The array configuration is asshown in FIG. 6, which depicts the locations of the transducer housings122 mounted to the transducer mounting plate 124.

With continued reference to FIG. 5, a separator 126, in spaced relationto the transducer discs 120, is retained between a bottom retainingplate 128 and a top retaining plate 130. Gas delivery tubes 132 areconnected to gas distribution manifolds 134, which have gas deliveryports 136. The gas distribution manifolds 134 are housed within agenerator body 138 that is covered by generator lid 140. A transducerdriver 144, having circuitry for driving the transducer discs 120, iselectronically connected with the transducer discs 120 via electricalcables 146.

During operation of the aerosol generator 106, as shown in FIG. 5, thetransducer discs 120 are activated by the transducer driver 144 via theelectrical cables 146. The transducers preferably vibrate at a frequencyof from about 1 MHz to about 5 MHz, more preferably from about 1.5 MHzto about 3 MHz. Frequently used frequencies are at about 1.6 MHz andabout 2.4 MHz. Furthermore, all of the transducer discs 110 should beoperating at substantially the same frequency when an aerosol with anarrow droplet size distribution is desired. This is important becausecommercially available transducers can vary significantly in thickness,sometimes by as much as 10%. It is preferred, however, that thetransducer discs 120 operate at frequencies within a range of 5% aboveand below the median transducer frequency, more preferably within arange of 2.5%, and most preferably within a range of 1%. This can beaccomplished by careful selection of the transducer discs 120 so thatthey all preferably have thicknesses within 5% of the median transducerthickness, more preferably within 2.5%, and most preferably within 1%.

Liquid feed 102 enters through a feed inlet 148 and flows through flowchannels 150 to exit through feed outlet 152. An ultrasonicallytransmissive fluid, typically water, enters through a water inlet 154 tofill a water bath volume 156 and flow through flow channels 158 to exitthrough a water outlet 160. A proper flow rate of the ultrasonicallytransmissive fluid is necessary to cool the transducer discs 120 and toprevent overheating of the ultrasonically transmissive fluid. Ultrasonicsignals from the transducer discs 120 are transmitted, via theultrasonically transmissive fluid, across the water bath volume 156, andultimately across the separator 126, to the liquid feed 102 in flowchannels 150.

The ultrasonic signals from the ultrasonic transducer discs 120 causeatomization cones 162 to develop in the liquid feed 102 at locationscorresponding with the transducer discs 120. Carrier gas 104 isintroduced into the gas delivery tubes 132 and delivered to the vicinityof the atomization cones 162 via gas delivery ports 136. Jets of carriergas exit the gas delivery ports 136 in a direction so as to impinge onthe atomization cones 162, thereby sweeping away atomized droplets ofthe liquid feed 102 that are being generated from the atomization cones162 and creating the aerosol 108, which exits the aerosol generator 106through an aerosol exit opening 164.

Efficient use of the carrier gas 104 is an important aspect of theaerosol generator 106. The embodiment of the aerosol generator 106 shownin FIG. 5 includes two gas exit ports per atomization cone 162, with thegas ports being positioned above the liquid medium 102 over troughs thatdevelop between the atomization cones 162, such that the exiting carriergas 104 is horizontally directed at the surface of the atomization cones162, thereby efficiently distributing the carrier gas 104 to criticalportions of the liquid feed 102 for effective and efficient sweepingaway of droplets as they form about the ultrasonically energizedatomization cones 162. Furthermore, it is preferred that at least aportion of the opening of each of the gas delivery ports 136, throughwhich the carrier gas exits the gas delivery tubes, should be locatedbelow the top of the atomization cones 162 at which the carrier gas 104is directed. This relative placement of the gas delivery ports 136 isvery important to efficient use of carrier gas 104. Orientation of thegas delivery ports 136 is also important. Preferably, the gas deliveryports 136 are positioned to horizontally direct jets of the carrier gas104 at the atomization cones 162. The aerosol generator 106 permitsgeneration of the aerosol 108 with heavy loading with droplets of thecarrier liquid 102, unlike aerosol generator designs that do notefficiently focus gas delivery to the locations of droplet formation.

Another important feature of the aerosol generator 106, as shown in FIG.5, is the use of the separator 126, which protects the transducer discs120 from direct contact with the liquid feed 102, which is often highlycorrosive. The height of the separator 126 above the top of thetransducer discs 120 should normally be kept as small as possible, andis often in the range of from about 1 centimeter to about 2 centimeters.The top of the liquid feed 102 in the flow channels above the tops ofthe ultrasonic transducer discs 120 is typically in a range of fromabout 2 centimeters to about 5 centimeters, whether or not the aerosolgenerator includes the separator 126, with a distance of about 3 to 4centimeters being preferred. Although the aerosol generator 106 could bemade without the separator 126, in which case the liquid feed 102 wouldbe in direct contact with the transducer discs 120, the highly corrosivenature of the liquid feed 102 can often cause premature failure of thetransducer discs 120. The use of the separator 126, in combination withuse of the ultrasonically transmissive fluid in the water bath volume156 to provide ultrasonic coupling, significantly extending the life ofthe ultrasonic transducers 120. One disadvantage of using the separator126, however, is that the rate of droplet production from theatomization cones 162 is reduced, often by a factor of two or more,relative to designs in which the liquid feed 102 is in direct contactwith the ultrasonic transducer discs 102. Even with the separator 126,however, the aerosol generator 106 used with the present invention iscapable of producing a high quality aerosol with heavy droplet loading,as previously discussed. Suitable materials for the separator 126include, for example, polyamides (such as Kapton™ membranes from DuPont)and other polymer materials, glass, and plexiglass. The mainrequirements for the separator 126 are that it be ultrasonicallytransmissive, corrosion resistant and impermeable.

One alternative to using the separator 126 is to bind acorrosion-resistant protective coating onto the surface of theultrasonic transducer discs 120, thereby preventing the liquid feed 102from contacting the surface of the ultrasonic transducer discs 120. Whenthe ultrasonic transducer discs 120 have a protective coating, theaerosol generator 106 will typically be constructed without the waterbath volume 156 and the liquid feed 102 will flow directly over theultrasonic transducer discs 120. Examples of such protective coatingmaterials include platinum, gold, TEFLON™, epoxies and various plastics.Such coating typically significantly extends transducer life. Also, whenoperating without the separator 126, the aerosol generator 106 willtypically produce the aerosol 108 with a much higher droplet loadingthan when the separator 126 is used.

One surprising finding with operation of the aerosol generator 106 ofthe present invention is that the droplet loading in the aerosol may beaffected by the temperature of the liquid feed 102. It has been foundthat when the liquid feed 102 includes an aqueous liquid at an elevatedtemperature, the droplet loading increases significantly. Thetemperature of the liquid feed 102 is preferably higher than about 30°C., more preferably higher than about 35° C. and most preferably higherthan about 40° C. If the temperature becomes too high, however, it canhave a detrimental effect on droplet loading in the aerosol 108.Therefore, the temperature of the liquid feed 102 from which the aerosol108 is made should generally be lower than about 50° C., and preferablylower than about 45° C. The liquid feed 102 may be maintained at thedesired temperature in any suitable fashion. For example, the portion ofthe aerosol generator 106 where the liquid feed 102 is converted to theaerosol 108 could be maintained at a constant elevated temperature.Alternatively, the liquid feed 102 could be delivered to the aerosolgenerator 106 from a constant temperature bath maintained separate fromthe aerosol generator 106. When the ultrasonic generator 106 includesthe separator 126, the ultrasonically transmissive fluid adjacent theultrasonic transducer disks 120 are preferably also at an elevatedtemperature in the ranges just discussed for the liquid feed 102.

The design for the aerosol generator 106 based on an array of ultrasonictransducers is versatile and is easily modified to accommodate differentgenerator sizes for different specialty applications. The aerosolgenerator 106 may be designed to include a plurality of ultrasonictransducers in any convenient number. Even for smaller scale production,however, the aerosol generator 106 preferably has at least nineultrasonic transducers, more preferably at least 16 ultrasonictransducers, and even more preferably at least 25 ultrasonictransducers. For larger scale production, however, the aerosol generator106 includes at least 40 ultrasonic transducers, more preferably atleast 100 ultrasonic transducers, and even more preferably at least 400ultrasonic transducers. In some large volume applications, the aerosolgenerator may have at least 1000 ultrasonic transducers.

FIGS. 7–24 show component designs for an aerosol generator 106 includingan array of 400 ultrasonic transducers. Referring first to FIGS. 7 and8, the transducer mounting plate 124 is shown with a design toaccommodate an array of 400 ultrasonic transducers, arranged in foursubarrays of 100 ultrasonic transducers each. The transducer mountingplate 124 has integral vertical walls 172 for containing theultrasonically transmissive fluid, typically water, in a water bathsimilar to the water bath volume 156 described previously with referenceto FIG. 5.

As shown in FIGS. 7 and 8, four hundred transducer mounting receptacles174 are provided in the transducer mounting plate 124 for mountingultrasonic transducers for the desired array. With reference to FIG. 9,the profile of an individual transducer mounting receptacle 174 isshown. A mounting seat 176 accepts an ultrasonic transducer formounting, with a mounted ultrasonic transducer being held in place viascrew holes 178. Opposite the mounting receptacle 176 is a flaredopening 180 through which an ultrasonic signal may be transmitted forthe purpose of generating the aerosol 108, as previously described withreference to FIG. 5.

A preferred transducer mounting configuration, however, is shown in FIG.10 for another configuration for the transducer mounting plate 124. Asseen in FIG. 10, an ultrasonic transducer disc 120 is mounted to thetransducer mounting plate 124 by use of a compression screw 177 threadedinto a threaded receptacle 179. The compression screw 177 bears againstthe ultrasonic transducer disc 120, causing an o-ring 181, situated inan o-ring seat 182 on the transducer mounting plate, to be compressed toform a seal between the transducer mounting plate 124 and the ultrasonictransducer disc 120. This type of transducer mounting is particularlypreferred when the ultrasonic transducer disc 120 includes a protectivesurface coating, as discussed previously, because the seal of the o-ringto the ultrasonic transducer disc 120 will be inside of the outer edgeof the protective seal, thereby preventing liquid from penetrating underthe protective surface coating from the edges of the ultrasonictransducer disc 120.

Referring now to FIG. 11, the bottom retaining plate 128 for a 400transducer array is shown having a design for mating with the transducermounting plate 124 (shown in FIGS. 7–8). The bottom retaining plate 128has eighty openings 184, arranged in four subgroups 186 of twentyopenings 184 each. Each of the openings 184 corresponds with five of thetransducer mounting receptacles 174 (shown in FIGS. 7 and 8) when thebottom retaining plate 128 is mated with the transducer mounting plate124 to create a volume for a water bath between the transducer mountingplate 124 and the bottom retaining plate 128. The openings 184,therefore, provide a pathway for ultrasonic signals generated byultrasonic transducers to be transmitted through the bottom retainingplate.

Referring now to FIGS. 12 and 13, a liquid feed box 190 for a 400transducer array is shown having the top retaining plate 130 designed tofit over the bottom retaining plate 128 (shown in FIG. 11), with aseparator 126 (not shown) being retained between the bottom retainingplate 128 and the top retaining plate 130 when the aerosol generator 106is assembled. The liquid feed box 190 also includes vertically extendingwalls 192 for containing the liquid feed 102 when the aerosol generatoris in operation. Also shown in FIGS. 12 and 13 is the feed inlet 148 andthe feed outlet 152. An adjustable weir 198 determines the level ofliquid feed 102 in the liquid feed box 190 during operation of theaerosol generator 106.

The top retaining plate 130 of the liquid feed box 190 has eightyopenings 194 therethrough, which are arranged in four subgroups 196 oftwenty openings 194 each. The openings 194 of the top retaining plate130 correspond in size with the openings 184 of the bottom retainingplate 128 (shown in FIG. 11). When the aerosol generator 106 isassembled, the openings 194 through the top retaining plate 130 and theopenings 184 through the bottom retaining plate 128 are aligned, withthe separator 126 positioned therebetween, to permit transmission ofultrasonic signals when the aerosol generator 106 is in operation.

Referring now to FIGS. 12–14, a plurality of gas tube feed-through holes202 extend through the vertically extending walls 192 to either side ofthe assembly including the feed inlet 148 and feed outlet 152 of theliquid feed box 190. The gas tube feed-through holes 202 are designed topermit insertion therethrough of gas tubes 208 of a design as shown inFIG. 14. When the aerosol generator 106 is assembled, a gas tube 208 isinserted through each of the gas tube feed-through holes 202 so that gasdelivery ports 136 in the gas tube 208 will be properly positioned andaligned adjacent the openings 194 in the top retaining plate 130 fordelivery of gas to atomization cones that develop in the liquid feed box190 during operation of the aerosol generator 106. The gas deliveryports 136 are typically holes having a diameter of from about 1.5millimeters to about 3.5 millimeters.

Referring now to FIG. 15, a partial view of the liquid feed box 190 isshown with gas tubes 208A, 208B and 208C positioned adjacent to theopenings 194 through the top retaining plate 130. Also shown in FIG. 15are the relative locations that ultrasonic transducer discs 120 wouldoccupy when the aerosol generator 106 is assembled. As seen in FIG. 15,the gas tube 208A, which is at the edge of the array, has five gasdelivery ports 136. Each of the gas delivery ports 136 is positioned todivert carrier gas 104 to a different one of atomization cones thatdevelop over the array of ultrasonic transducer discs 120 when theaerosol generator 106 is operating. The gas tube 208B, which is one rowin from the edge of the array, is a shorter tube that has ten gasdelivery ports 136, five each on opposing sides of the gas tube 208B.The gas tube 208B, therefore, has gas delivery ports 136 for deliveringgas to atomization cones corresponding with each of ten ultrasonictransducer discs 120. The third gas tube, 208C, is a longer tube thatalso has ten gas delivery ports 136 for delivering gas to atomizationcones corresponding with ten ultrasonic transducer discs 120. The designshown in FIG. 15, therefore, includes one gas delivery port perultrasonic transducer disc 120. Although this is a lower density of gasdelivery ports 136 than for the embodiment of the aerosol generator 106shown in FIG. 5, which includes two gas delivery ports per ultrasonictransducer disc 120, the design shown in FIG. 15 is, nevertheless,capable of producing a dense, high-quality aerosol without unnecessarywaste of gas.

Referring now to FIG. 16, the flow of carrier gas 104 relative toatomization cones 162 during operation of the aerosol generator 106having a gas distribution configuration to deliver carrier gas 104 fromgas delivery ports on both sides of the gas tubes 208, as was shown forthe gas tubes 208A, 208B and 208C in the gas distribution configurationshown in FIG. 14. The carrier gas 104 sweeps both directions from eachof the gas tubes 208.

An alternative, and preferred, flow for carrier gas 104 is shown in FIG.17. As shown in FIG. 17, carrier gas 104 is delivered from only one sideof each of the gas tubes 208. This results in a sweep of carrier gasfrom all of the gas tubes 208 toward a central area 212. This results ina more uniform flow pattern for aerosol generation that maysignificantly enhance the efficiency with which the carrier gas 104 isused to produce an aerosol. The aerosol that is generated, therefore,tends to be more heavily loaded with liquid droplets.

Another configuration for distributing carrier gas in the aerosolgenerator 106 is shown in FIGS. 18 and 19. In this configuration, thegas tubes 208 are hung from a gas distribution plate 216 adjacent gasflow holes 218 through the gas distribution plate 216. In the aerosolgenerator 106, the gas distribution plate 216 would be mounted above theliquid feed, with the gas flow holes positioned to each correspond withan underlying ultrasonic transducer. Referring specifically to FIG. 19,when the ultrasonic generator 106 is in operation, atomization cones 162develop through the gas flow holes 218, and the gas tubes 208 arelocated such that carrier gas 104 exiting from ports in the gas tubes208 impinge on the atomization cones and flow upward through the gasflow holes. The gas flow holes 218, therefore, act to assist inefficiently distributing the carrier gas 104 about the atomization cones162 for aerosol formation. It should be appreciated that the gasdistribution plates 218 can be made to accommodate any number of the gastubes 208 and gas flow holes 218. For convenience of illustration, theembodiment shown in FIGS. 18 and 19 shows a design having only two ofthe gas tubes 208 and only 16 of the gas flow holes 218. Also, it shouldbe appreciated that the gas distribution plate 216 could be used alone,without the gas tubes 208. In that case, a slight positive pressure ofcarrier gas 104 would be maintained under the gas distribution plate 216and the gas flow holes 218 would be sized to maintain the propervelocity of carrier gas 104 through the gas flow holes 218 for efficientaerosol generation. Because of the relative complexity of operating inthat mode, however, it is not preferred.

Aerosol generation may also be enhanced through mounting of ultrasonictransducers at a slight angle and directing the carrier gas at resultingatomization cones such that the atomization cones are tilting in thesame direction as the direction of flow of carrier gas. Referring toFIG. 20, an ultrasonic transducer disc 120 is shown. The ultrasonictransducer disc 120 is tilted at a tilt angle 114 (typically less than10 degrees), so that the atomization cone 162 will also have a tilt. Itis preferred that the direction of flow of the carrier gas 104 directedat the atomization cone 162 is in the same direction as the tilt of theatomization cone 162.

Referring now to FIGS. 21 and 22, a gas manifold 220 is shown fordistributing gas to the gas tubes 208 in a 400 transducer array design.The gas manifold 220 includes a gas distribution box 222 and pipingstubs 224 for connection with gas tubes 208 (shown in FIG. 14). Insidethe gas distribution box 222 are two gas distribution plates 226 thatform a flow path to assist in distributing the gas equally throughoutthe gas distribution box 222, to promote substantially equal delivery ofgas through the piping stubs 224. The gas manifold 220, as shown inFIGS. 21 and 22, is designed to feed eleven gas tubes 208. For the 400transducer design, a total of four gas manifolds 220 are required.

Referring now to FIGS. 23 and 24, the generator lid 140 is shown for a400 transducer array design. The generator lid 140 mates with and coversthe liquid feed box 190 (shown in FIGS. 12 and 13). The generator lid140, as shown in FIGS. 23 and 24, has a hood design to permit easycollection of the aerosol 108 without subjecting droplets in the aerosol108 to sharp edges on which droplets may coalesce and be lost, andpossibly interfere with the proper operation of the aerosol generator106. When the aerosol generator 106 is in operation, the aerosol 108would be withdrawn via the aerosol exit opening 164 through thegenerator cover 140.

The design and apparatus of the aerosol generator 106 described withreference to FIGS. 5–24, as well as a facility including other processequipment described herein for carrying out the process of the presentinvention for making powders are within the scope of the presentinvention.

Although the aerosol generator 106 produces a high quality aerosol 108having a high droplet loading, it is often desirable to furtherconcentrate the aerosol 108 prior to introduction into the furnace 110.Referring now to FIG. 25, a process flow diagram is shown for oneembodiment of the present invention involving such concentration of theaerosol 108. As shown in FIG. 25, the aerosol 108 from the aerosolgenerator 106 is sent to an aerosol concentrator 236 where excesscarrier gas 238 is withdrawn from the aerosol 108 to produce aconcentrated aerosol 240, which is then fed to the furnace 110.

The aerosol concentrator 236 typically includes one or more virtualimpactors capable of concentrating droplets in the aerosol 108 by afactor of greater than about 2, preferably by a factor of greater thanabout 5, and more preferably by a factor of greater than about 10, toproduce the concentrated aerosol 240. According to the presentinvention, the concentrated aerosol 240 should typically contain greaterthan about 1×10⁷ droplets per cubic centimeter, and more preferably fromabout 5×10⁷ to about 5×10⁸ droplets per cubic centimeter. Aconcentration of about 1×10⁸ droplets per cubic centimeter of theconcentrated aerosol is particularly preferred, because when theconcentrated aerosol 240 is loaded more heavily than that, then thefrequency of collisions between droplets becomes large enough to impairthe properties of the concentrated aerosol 240, resulting in potentialcontamination of the particulate product 116 with an undesirably largequantity of over-sized particles. For example, if the aerosol 108 has aconcentration of about 1×10⁷ droplets per cubic centimeter, and theaerosol concentrator 236 concentrates droplets by a factor of 10, thenthe concentrated aerosol 240 will have a concentration of about 1×10⁸droplets per cubic centimeter. Stated another way, for example, when theaerosol generator generates the aerosol 108 with a droplet loading ofabout 0.167 milliliters liquid feed 102 per liter of carrier gas 104,the concentrated aerosol 240 would be loaded with about 1.67 millilitersof liquid feed 102 per liter of carrier gas 104, assuming the aerosol108 is concentrated by a factor of 10.

Having a high droplet loading in aerosol feed to the furnace providesthe important advantage of reducing the heating demand on the furnace110 and the size of flow conduits required through the furnace. Also,other advantages of having a dense aerosol include a reduction in thedemands on cooling and particle collection components, permittingsignificant equipment and operational savings. Furthermore, as systemcomponents are reduced in size, powder holdup within the system isreduced, which is also desirable. Concentration of the aerosol streamprior to entry into the furnace 110, therefore, provides a substantialadvantage relative to processes that utilize less concentrated aerosolstreams.

The excess carrier gas 238 that is removed in the aerosol concentrator236 typically includes extremely small droplets that are also removedfrom the aerosol 108. Preferably, the droplets removed with the excesscarrier gas 238 have a weight average size of smaller than about 1.5microns, and more preferably smaller than about 1 micron and thedroplets retained in the concentrated aerosol 240 have an averagedroplet size of larger than about 2 microns. For example, a virtualimpactor sized to treat an aerosol stream having a weight averagedroplet size of about three microns might be designed to remove with theexcess carrier gas 238 most droplets smaller than about 1.5 microns insize. Other designs are also possible. When using the aerosol generator106 with the present invention, however, the loss of these very smalldroplets in the aerosol concentrator 236 will typically constitute nomore than about 10 percent by weight, and more preferably no more thanabout 5 percent by weight, of the droplets originally in the aerosolstream that is fed to the concentrator 236. Although the aerosolconcentrator 236 is useful in some situations, it is normally notrequired with the process of the present invention, because the aerosolgenerator 106 is capable, in most circumstances, of generating anaerosol stream that is sufficiently dense. So long as the aerosol streamcoming out of the aerosol generator 102 is sufficiently dense, it ispreferred that the aerosol concentrator not be used. It is a significantadvantage of the present invention that the aerosol generator 106normally generates such a dense aerosol stream that the aerosolconcentrator 236 is not needed. Therefore, the complexity of operationof the aerosol concentrator 236 and accompanying liquid losses maytypically be avoided.

It is important that the aerosol stream (whether it has beenconcentrated or not) that is fed to the furnace 110 have a high dropletflow rate and high droplet loading as would be required for mostindustrial applications. With the present invention, the aerosol streamfed to the furnace preferably includes a droplet flow of greater thanabout 0.5 liters per hour, more preferably greater than about 2 litersper hour, still more preferably greater than about 5 liters per hour,even more preferably greater than about 10 liters per hour, particularlygreater than about 50 liters per hour and most preferably greater thanabout 100 liters per hour; and with the droplet loading being typicallygreater than about 0.04 milliliters of droplets per liter of carriergas, preferably greater than about 0.083 milliliters of droplets perliter of carrier gas 104, more preferably greater than about 0.167milliliters of droplets per liter of carrier gas 104, still morepreferably greater than about 0.25 milliliters of droplets per liter ofcarrier gas 104, particularly greater than about 0.33 milliliters ofdroplets per liter of carrier gas 104 and most preferably greater thanabout 0.83 milliliters of droplets per liter of carrier gas 104.

One embodiment of a virtual impactor that could be used as the aerosolconcentrator 236 will now be described with reference to FIGS. 26–32. Avirtual impactor 246 includes an upstream plate assembly 248 (detailsshown in FIGS. 27–29) and a downstream plate assembly 250 (details shownin FIGS. 30–32), with a concentrating chamber 262 located between theupstream plate assembly 248 and the downstream plate assembly 250.

Through the upstream plate assembly 248 are a plurality of verticallyextending inlet slits 254. The downstream plate assembly 250 includes aplurality of vertically extending exit slits 256 that are in alignmentwith the inlet slits 254. The exit slits 256 are, however, slightlywider than the inlet slits 254. The downstream plate assembly 250 alsoincludes flow channels 258 that extend substantially across the width ofthe entire downstream plate assembly 250, with each flow channel 258being adjacent to an excess gas withdrawal port 260.

During operation, the aerosol 108 passes through the inlet slits 254 andinto the concentrating chamber 262. Excess carrier gas 238 is withdrawnfrom the concentrating chamber 262 via the excess gas withdrawal ports260. The withdrawn excess carrier gas 238 then exits via a gas duct port264. That portion of the aerosol 108 that is not withdrawn through theexcess gas withdrawal ports 260 passes through the exit slits 256 andthe flow channels 258 to form the concentrated aerosol 240. Thosedroplets passing across the concentrating chamber 262 and through theexit slits 256 are those droplets of a large enough size to havesufficient momentum to resist being withdrawn with the excess carriergas 238.

As seen best in FIGS. 27–32, the inlet slits 254 of the upstream plateassembly 248 include inlet nozzle extension portions 266 that extendoutward from the plate surface 268 of the upstream plate assembly 248.The exit slits 256 of the downstream plate assembly 250 include exitnozzle extension portions 270 extending outward from a plate surface 272of the downstream plate assembly 250. These nozzle extension portions266 and 270 are important for operation of the virtual impactor 246,because having these nozzle extension portions 266 and 270 permits avery close spacing to be attained between the inlet slits 254 and theexit slits 256 across the concentrating chamber 262, while alsoproviding a relatively large space in the concentrating chamber 262 tofacilitate efficient removal of the excess carrier gas 238.

Also as best seen in FIGS. 27–32, the inlet slits 254 have widths thatflare outward toward the side of the upstream plate assembly 248 that isfirst encountered by the aerosol 108 during operation. This flaredconfiguration reduces the sharpness of surfaces encountered by theaerosol 108, reducing the loss of aerosol droplets and potentialinterference from liquid buildup that could occur if sharp surfaces werepresent. Likewise, the exit slits 256 have a width that flares outwardtowards the flow channels 258, thereby allowing the concentrated aerosol240 to expand into the flow channels 258 without encountering sharpedges that could cause problems.

As noted previously, both the inlet slits 254 of the upstream plateassembly 248 and the exit slits 256 of the downstream plate assembly 250are vertically extending. This configuration is advantageous forpermitting liquid that may collect around the inlet slits 254 and theexit slits 256 to drain away. The inlet slits 254 and the exit slits 256need not, however, have a perfectly vertical orientation. Rather, it isoften desirable to slant the slits backward (sloping upward and away inthe direction of flow) by about five to ten degrees relative tovertical, to enhance draining of liquid off of the upstream plateassembly 248 and the downstream plate assembly 250. This drainagefunction of the vertically extending configuration of the inlet slits254 and the outlet slits 256 also inhibits liquid build-up in thevicinity of the inlet slits 248 and the exit slits 250, which liquidbuild-up could result in the release of undesirably large droplets intothe concentrated aerosol 240.

As discussed previously, the aerosol generator 106 of the presentinvention produces a concentrated, high quality aerosol of micro-sizeddroplets having a relatively narrow size distribution. It has beenfound, however, that for many applications the process of the presentinvention is significantly enhanced by further classifying by size thedroplets in the aerosol 108 prior to introduction of the droplets intothe furnace 110. In this manner, the size and size distribution ofparticles in the particulate product 116 are further controlled.

Referring now to FIG. 33, a process flow diagram is shown for oneembodiment of the process of the present invention including suchdroplet classification. As shown in FIG. 33, the aerosol 108 from theaerosol generator 106 goes to a droplet classifier 280 where oversizeddroplets are removed from the aerosol 108 to prepare a classifiedaerosol 282. Liquid 284 from the oversized droplets that are beingremoved is drained from the droplet classifier 280. This drained liquid284 may advantageously be recycled for use in preparing additionalliquid feed 102.

Any suitable droplet classifier may be used for removing droplets abovea predetermined size. For example, a cyclone could be used to removeover-size droplets. A preferred droplet classifier for manyapplications, however, is an impactor. One embodiment of an impactor foruse with the present invention will now be described with reference toFIGS. 34–38.

As seen in FIG. 34, an impactor 288 has disposed in a flow conduit 286 aflow control plate 290 and an impactor plate assembly 292. The flowcontrol plate 290 is conveniently mounted on a mounting plate 294.

The flow control plate 290 is used to channel the flow of the aerosolstream toward the impactor plate assembly 292 in a manner withcontrolled flow characteristics that are desirable for proper impactionof oversize droplets on the impactor plate assembly 292 for removalthrough the drains 296 and 314. One embodiment of the flow control plate290 is shown in FIG. 35. The flow control plate 290 has an array ofcircular flow ports 296 for channeling flow of the aerosol 108 towardsthe impactor plate assembly 292 with the desired flow characteristics.

Details of the mounting plate 294 are shown in FIG. 36. The mountingplate 294 has a mounting flange 298 with a large diameter flow opening300 passing therethrough to permit access of the aerosol 108 to the flowports 296 of the flow control plate 290 (shown in FIG. 35).

Referring now to FIGS. 37 and 38, one embodiment of an impactor plateassembly 292 is shown. The impactor plate assembly 292 includes animpactor plate 302 and mounting brackets 304 and 306 used to mount theimpactor plate 302 inside of the flow conduit 286. The impactor plate302 and the flow channel plate 290 are designed so that droplets largerthan a predetermined size will have momentum that is too large for thoseparticles to change flow direction to navigate around the impactor plate302.

During operation of the impactor 288, the aerosol 108 from the aerosolgenerator 106 passes through the upstream flow control plate 290. Mostof the droplets in the aerosol navigate around the impactor plate 302and exit the impactor 288 through the downstream flow control plate 290in the classified aerosol 282. Droplets in the aerosol 108 that are toolarge to navigate around the impactor plate 302 will impact on theimpactor plate 302 and drain through the drain 296 to be collected withthe drained liquid 284 (as shown in FIG. 34).

The configuration of the impactor plate 302 shown in FIG. 33 representsonly one of many possible configurations for the impactor plate 302. Forexample, the impactor 288 could include an upstream flow control plate290 having vertically extending flow slits therethrough that are offsetfrom vertically extending flow slits through the impactor plate 302,such that droplets too large to navigate the change in flow due to theoffset of the flow slits between the flow control plate 290 and theimpactor plate 302 would impact on the impactor plate 302 to be drainedaway. Other designs are also possible.

In a preferred embodiment of the present invention, the dropletclassifier 280 is typically designed to remove droplets from the aerosol108 that are larger than about 15 microns in size, more preferably toremove droplets larger than about 10 microns in size, even morepreferably to remove droplets of a size larger than about 8 microns insize and most preferably to remove droplets larger than about 5 micronsin size. The droplet classification size in the droplet classifier ispreferably smaller than about 15 microns, more preferably smaller thanabout 10 microns, even more preferably smaller than about 8 microns andmost preferably smaller than about 5 microns. The classification size,also called the classification cut point, is that size at which half ofthe droplets of that size are removed and half of the droplets of thatsize are retained. Depending upon the specific application, however, thedroplet classification size may be varied, such as by changing thespacing between the impactor plate 302 and the flow control plate 290 orincreasing or decreasing aerosol velocity through the jets in the flowcontrol plate 290. Because the aerosol generator 106 of the presentinvention initially produces a high quality aerosol 108, having arelatively narrow size distribution of droplets, typically less thanabout 30 weight percent of liquid feed 102 in the aerosol 108 is removedas the drain liquid 284 in the droplet classifier 288, with preferablyless than about 25 weight percent being removed, even more preferablyless than about 20 weight percent being removed and most preferably lessthan about 15 weight percent being removed. Minimizing the removal ofliquid feed 102 from the aerosol 108 is particularly important forcommercial applications to increase the yield of high qualityparticulate product 116. It should be noted, however, that because ofthe superior performance of the aerosol generator 106, it is frequentlynot required to use an impactor or other droplet classifier to obtain adesired absence of oversize droplets to the furnace. This is a majoradvantage, because the added complexity and liquid losses accompanyinguse of an impactor may often be avoided with the process of the presentinvention.

Sometimes it is desirable to use both the aerosol concentrator 236 andthe droplet classifier 280 to produce an extremely high quality aerosolstream for introduction into the furnace for the production of particlesof highly controlled size and size distribution. Referring now to FIG.39, one embodiment of the present invention is shown incorporating boththe virtual impactor 246 and the impactor 288. Basic components of thevirtual impactor 246 and the impactor 288, as shown in FIG. 39, aresubstantially as previously described with reference to FIGS. 26–38. Asseen in FIG. 39, the aerosol 108 from the aerosol generator 106 is fedto the virtual impactor 246 where the aerosol stream is concentrated toproduce the concentrated aerosol 240. The concentrated aerosol 240 isthen fed to the impactor 288 to remove large droplets therefrom andproduce the classified aerosol 282, which may then be fed to the furnace110. Also, it should be noted that by using both a virtual impactor andan impactor, both undesirably large and undesirably small droplets areremoved, thereby producing a classified aerosol with a very narrowdroplet size distribution. Also, the order of the aerosol concentratorand the aerosol classifier could be reversed, so that the aerosolconcentrator 236 follows the aerosol classifier 280.

One important feature of the design shown in FIG. 39 is theincorporation of drains 310, 312, 314, 316 and 296 at strategiclocations. These drains are extremely important for industrial-scaleparticle production because buildup of liquid in the process equipmentcan significantly impair the quality of the particulate product 116 thatis produced. In that regard, drain 310 drains liquid away from the inletside of the first plate assembly 248 of the virtual impactor 246. Drain312 drains liquid away from the inside of the concentrating chamber 262in the virtual impactor 246 and drain 314 removes liquid that depositsout of the excess carrier gas 238. Drain 316 removes liquid from thevicinity of the inlet side of the flow control plate 290 of theimpactor, while the drain 296 removes liquid from the vicinity of theimpactor plate 302. Without these drains 310, 312, 314, 316 and 296, theperformance of the apparatus shown in FIG. 39 would be significantlyimpaired. All liquids drained in the drains 310, 312, 314, 316 and 296may advantageously be recycled for use to prepare the liquid feed 102.

With some applications of the process of the present invention, it maybe possible to collect the particles 112 directly from the output of thefurnace 110. More often, however, it will be desirable to cool theparticles 112 exiting the furnace 110 prior to collection of theparticles 112 in the particle collector 114. Referring now to FIG. 40,one embodiment of the process of the present invention is shown in whichthe particles 112 exiting the furnace 110 are sent to a particle cooler320 to produce a cooled particle stream 322, which is then feed to theparticle collector 114. Although the particle cooler 320 may be anycooling apparatus capable of cooling the particles 112 to the desiredtemperature for introduction into the particle collector 114,traditional heat exchanger designs are not preferred. This is because atraditional heat exchanger design ordinarily directly subjects theaerosol stream, in which the hot particles 112 are suspended, to coolsurfaces. In that situation, significant losses of the particles 112occur due to thermophoretic deposition of the hot particles 112 on thecool surfaces of the heat exchanger. According to the present invention,a gas quench apparatus is provided for use as the particle cooler 320that significantly reduces thermophoretic losses compared to atraditional heat exchanger.

Referring now to FIGS. 41–43, one embodiment of a gas quench cooler 330is shown. The gas quench cooler includes a perforated conduit 332 housedinside of a cooler housing 334 with an annular space 336 located betweenthe cooler housing 334 and the perforated conduit 332. In fluidcommunication with the annular space 336 is a quench gas inlet box 338,inside of which is disposed a portion of an aerosol outlet conduit 340.The perforated conduit 332 extends between the aerosol outlet conduit340 and an aerosol inlet conduit 342. Attached to an opening into thequench gas inlet box 338 are two quench gas feed tubes 344. Referringspecifically to FIG. 43, the perforated tube 332 is shown. Theperforated tube 332 has a plurality of openings 345. The openings 345,when the perforated conduit 332 is assembled into the gas quench cooler330, permit the flow of quench gas 346 from the annular space 336 intothe interior space 348 of the perforated conduit 332. Although theopenings 345 are shown as being round holes, any shape of opening couldbe used, such as slits. Also, the perforated conduit 332 could be aporous screen. Two heat radiation shields 347 prevent downstream radiantheating from the furnace. In most instances, however, it will not benecessary to include the heat radiation shields 347, because downstreamradiant heating from the furnace is normally not a significant problem.Use of the heat radiation shields 347 is not preferred due toparticulate losses that accompany their use.

With continued reference to FIGS. 41–43, operation of the gas quenchcooler 330 will now be described. During operation, the particles 112,carried by and dispersed in a gas stream, enter the gas quench cooler330 through the aerosol inlet conduit 342 and flow into the interiorspace 348 of perforated conduit 332. Quench gas 346 is introducedthrough the quench gas feed tubes 344 into the quench gas inlet box 338.Quench gas 346 entering the quench gas inlet box 338 encounters theouter surface of the aerosol outlet conduit 340, forcing the quench gas346 to flow, in a spiraling, swirling manner, into the annular space336, where the quench gas 346 flows through the openings 345 through thewalls of the perforated conduit 332. Preferably, the gas 346 retainssome swirling motion even after passing into the interior space 348. Inthis way, the particles 112 are quickly cooled with low losses ofparticles to the walls of the gas quench cooler 330. In this manner, thequench gas 346 enters in a radial direction into the interior space 348of the perforated conduit 332 around the entire periphery, orcircumference, of the perforated conduit 332 and over the entire lengthof the perforated conduit 332. The cool quench gas 346 mixes with andcools the hot particles 112, which then exit through the aerosol outletconduit 340 as the cooled particle stream 322. The cooled particlestream 322 can then be sent to the particle collector 114 for particlecollection. The temperature of the cooled particle stream 322 iscontrolled by introducing more or less quench gas. Also, as shown inFIG. 41, the quench gas 346 is fed into the quench cooler 330 in counterflow to flow of the particles. Alternatively, the quench cooler could bedesigned so that the quench gas 346 is fed into the quench cooler inconcurrent flow with the flow of the particles 112. The amount of quenchgas 346 fed to the gas quench cooler 330 will depend upon the specificmaterial being made and the specific operating conditions. The quantityof quench gas 346 used, however, must be sufficient to reduce thetemperature of the aerosol steam including the particles 112 to thedesired temperature. Typically, the particles 112 are cooled to atemperature at least below about 200° C., and often lower. The onlylimitation on how much the particles 112 are cooled is that the cooledparticle stream 322 must be at a temperature that is above thecondensation temperature for water as another condensible vapor in thestream. The temperature of the cooled particle stream 322 is often at atemperature of from about 50° C. to about 120° C.

Because of the entry of quench gas 346 into the interior space 348 ofthe perforated conduit 322 in a radial direction about the entirecircumference and length of the perforated conduit 322, a buffer of thecool quench gas 346 is formed about the inner wall of the perforatedconduit 332, thereby significantly inhibiting the loss of hot particles112 due to thermophoretic deposition on the cool wall of the perforatedconduit 332. In operation, the quench gas 346 exiting the openings 345and entering into the interior space 348 should have a radial velocity(velocity inward toward the center of the circular cross-section of theperforated conduit 332) of larger than the thermophoretic velocity ofthe particles 112 inside the perforated conduit 332 in a directionradially outward toward the perforated wall of the perforated conduit332.

As seen in FIGS. 41–43, the gas quench cooler 330 includes a flow pathfor the particles 112 through the gas quench cooler of a substantiallyconstant cross-sectional shape and area. Preferably, the flow paththrough the gas quench cooler 330 will have the same cross-sectionalshape and area as the flow path through the furnace 110 and through theconduit delivering the aerosol 108 from the aerosol generator 106 to thefurnace 110. In one embodiment, however, it may be necessary to reducethe cross-sectional area available for flow prior to the particlecollector 114. This is the case, for example, when the particlecollector includes a cyclone for separating particles in the cooledparticle stream 322 from gas in the cooled particle stream 322. This isbecause of the high inlet velocity requirements into cyclone separators.

Referring now to FIG. 44, one embodiment of the gas quench cooler 330 isshown in combination with a cyclone separator 392. The perforatedconduit 332 has a continuously decreasing cross-sectional area for flowto increase the velocity of flow to the proper value for the feed tocyclone separator 392. Attached to the cyclone separator 392 is a bagfilter 394 for final clean-up of overflow from the cyclone separator392. Separated particles exit with underflow from the cyclone separator392 and may be collected in any convenient container. The use of cycloneseparation is particularly preferred for powder having a weight averagesize of larger than about 1 micron, although a series of cyclones maysometimes be needed to get the desired degree of separation. Cycloneseparation is particularly preferred for powders having a weight averagesize of larger than about 1.5 microns. Also, cyclone separation is bestsuited for high density materials. Preferably, when particles areseparated using a cyclone, the particles are of a composition withspecific gravity of greater than about 5.

In an additional embodiment, the process of the present invention canalso incorporate compositional modification of the particles 112 exitingthe furnace. Most commonly, the compositional modification will involveforming on the particles 112 a material phase that is different thanthat of the particles 112, such as by coating the particles 112 with acoating material. One embodiment of the process of the present inventionincorporating particle coating is shown in FIG. 45. As shown in FIG. 45,the particles 112 exiting from the furnace 110 go to a particle coater350 where a coating is placed over the outer surface of the particles112 to form coated particles 352, which are then sent to the particlecollector 114 for preparation of the particulate product 116.

In the particle coater 350, the particles 112 are coated using anysuitable particle coating technology, such as by gas-to-particleconversion. Preferably, however, the coating is accomplished by chemicalvapor deposition (CVD) and/or physical vapor deposition (PVD). In CVDcoating, one or more vapor phase coating precursors are reacted to forma surface coating on the particles 112. Preferred coatings deposited byCVD include oxides, such as silica, and elemental metals. In PVDcoating, coating material physically deposits on the surface of theparticles 112. Preferred coatings deposited by PVD include organicmaterials and elemental metals, such as elemental silver, copper andgold. Another possible surface coating method is surface conversion ofthe surface portion of the particles 112 by reaction with a vapor phasereactant to convert a surface portion of the particles to a differentmaterial than that originally contained in the particles 112. Althoughany suitable apparatus may be used for the particle coater 350, when agaseous coating feed involving coating precursors is used, such as forCVD and PVD, feed of the gaseous coating feed is introduced through acircumferentially perforated conduit, such as was described for thequench cooler 330 with reference to FIGS. 41–44. In some instances, thequench cooler 330 may also act as the particle coater 350, when coatingmaterial precursors are included in the quench gas 346.

With continued reference primarily to FIG. 45, in a preferredembodiment, when the particles 112 are coated according to the processof the present invention, the particles 112 are also manufactured viathe aerosol process of the present invention, as previously described.The process of the present invention can, however, be used to coatparticles that have been premanufactured by a different process, such asby a liquid precipitation route. When coating particles that have beenpremanufactured by a different route, such as by liquid precipitation,it is preferred that the particles remain in a dispersed state from thetime of manufacture to the time that the particles are introduced inslurry form into the aerosol generator 106 for preparation of theaerosol 108 to form the dry particles 112 in the furnace 110, whichparticles 112 can then be coated in the particle coater 350. Maintainingparticles in a dispersed state from manufacture through coating avoidsproblems associated with agglomeration and redispersion of particles ifparticles must be redispersed in the liquid feed 102 for feed to theaerosol generator 106. For example, for particles originallyprecipitated from a liquid medium, the liquid medium containing thesuspended precipitated particles could be used to form the liquid feed102 to the aerosol generator 106. It should be noted that the particlecoater 350 could be an integral extension of the furnace 110 or could bea separate piece of equipment.

In a further embodiment of the present invention, following preparationof the particles 112 in the furnace 110, the particles 112 may then bestructurally modified to impart desired physical properties prior toparticle collection. Referring now to FIG. 46, one embodiment of theprocess of the present invention is shown including such structuralparticle modification. The particles 112 exiting the furnace 110 go to aparticle modifier 360 where the particles are structurally modified toform modified particles 362, which are then sent to the particlecollector 114 for preparation of the particulate product 116. Theparticle modifier 360 is typically a furnace, such as an annealingfurnace, which may be integral with the furnace 110 or may be a separateheating device. Regardless, it is important that the particle modifier360 have temperature control that is independent of the furnace 110, sothat the proper conditions for particle modification may be providedseparate from conditions required of the furnace 110 to prepare theparticles 112. The particle modifier 360, therefore, typically providesa temperature controlled environment and necessary residence time toeffect the desired structural modification of the particles 112.

The structural modification that occurs in the particle modifier 360 maybe any modification to the crystalline structure or morphology of theparticles 112. For example, the particles 112 may be annealed in theparticle modifier 360 to densify the particles 112 or to recrystallizethe particles 112 into a polycrystalline or single crystalline form.Also, especially in the case of composite particles 112, the particlesmay be annealed for a sufficient time to permit redistribution withinthe particles 112 of different material phases.

The initial morphology of composite particles made in the furnace 110,according to the present invention, could take a variety of forms,depending upon the specified materials involved and the specificprocessing conditions. Examples of some possible composite particlemorphologies, manufacturable according to the present invention areshown in FIG. 47. These morphologies could be of the particles asinitially produced in the furnace 110 or that result from structuralmodification in the particle modifier 360. Furthermore, the compositeparticles could include a mixture of the morphological attributes shownin FIG. 47.

Referring now to FIG. 48, an embodiment of the apparatus of the presentinvention is shown that includes the aerosol generator 106 (in the formof the 400 transducer array design), the aerosol concentrator 236 (inthe form of a virtual impactor), the droplet classifier 280 (in the formof an impactor), the furnace 110, the particle cooler 320 (in the formof a gas quench cooler) and the particle collector 114 (in the form of abag filter). All process equipment components are connected viaappropriate flow conduits that are substantially free of sharp edgesthat could detrimentally cause liquid accumulations in the apparatus.Also, it should be noted that flex connectors 370 are used upstream anddownstream of the aerosol concentrator 236 and the droplet classifier280. By using the flex connectors 370, it is possible to vary the angleof slant of vertically extending slits in the aerosol concentrator 236and/or the droplet classifier 280. In this way, a desired slant for thevertically extending slits may be set to optimize the drainingcharacteristics off the vertically extending slits.

Aerosol generation with the process of the present invention has thusfar been described with respect to the ultrasonic aerosol generator. Useof the ultrasonic generator is preferred for the process of the presentinvention because of the extremely high quality and dense aerosolgenerated. In some instances, however, the aerosol generator for theprocess of the present invention may have a different design dependingupon the specific application. For example, when larger particles aredesired, such as those having a weight average size of larger than about3 microns, a spray nozzle atomizer may be preferred. Forsmaller-particle applications, however, and particularly for thoseapplications to produce particles smaller than about 3 microns, andpreferably smaller than about 2 microns in size, as is generally desiredwith the particles of the present invention, the ultrasonic generator,as described herein, is particularly preferred. In that regard, theultrasonic generator of the present invention is particularly preferredfor when making particles with a weight average size of from about 0.2micron to about 3 microns.

Although ultrasonic aerosol generators have been used for medicalapplications and home humidifiers, use of ultrasonic generators forspray pyrolysis particle manufacture has largely been confined tosmall-scale, experimental situations. The ultrasonic aerosol generatorof the present invention described with reference to FIGS. 5–24,however, is well suited for commercial production of high qualitypowders with a small average size and a narrow size distribution. Inthat regard, the aerosol generator produces a high quality aerosol, withheavy droplet loading and at a high rate of production. Such acombination of small droplet size, narrow size distribution, heavydroplet loading, and high production rate provide significant advantagesover existing aerosol generators that usually suffer from at least oneof inadequately narrow size distribution, undesirably low dropletloading, or unacceptably low production rate.

Through the careful and controlled design of the ultrasonic generator ofthe present invention, an aerosol may be produced typically havinggreater than about 70 weight percent (and preferably greater than about80 weight percent) of droplets in the size range of from about 1 micronto about 10 microns, preferably in a size range of from about 1 micronto about 5 microns and more preferably from about 2 microns to about 4microns. Also, the ultrasonic generator of the present invention iscapable of delivering high output rates of liquid feed in the aerosol.The rate of liquid feed, at the high liquid loadings previouslydescribed, is preferably greater than about 25 milliliters per hour pertransducer, more preferably greater than about 37.5 milliliters per hourper transducer, even more preferably greater than about 50 millilitersper hour per transducer and most preferably greater than about 100millimeters per hour per transducer. This high level of performance isdesirable for commercial operations and is accomplished with the presentinvention with a relatively simple design including a single precursorbath over an array of ultrasonic transducers. The ultrasonic generatoris made for high aerosol production rates at a high droplet loading, andwith a narrow size distribution of droplets. The generator preferablyproduces an aerosol at a rate of greater than about 0.5 liter per hourof droplets, more preferably greater than about 2 liters per hour ofdroplets, still more preferably greater than about 5 liters per hour ofdroplets, even more preferably greater than about 10 liters per hour ofdroplets and most preferably greater than about 40 liters per hour ofdroplets. For example, when the aerosol generator has a 400 transducerdesign, as described with reference to FIGS. 7–24, the aerosol generatoris capable of producing a high quality aerosol having high dropletloading as previously described, at a total production rate ofpreferably greater than about 10 liters per hour of liquid feed, morepreferably greater than about 15 liters per hour of liquid feed, evenmore preferably greater than about 20 liters per hour of liquid feed andmost preferably greater than about 40 liters per hour of liquid feed.

Under most operating conditions, when using such an aerosol generator,total particulate product produced is preferably greater than about 0.5gram per hour per transducer, more preferably greater than about 0.75gram per hour per transducer, even more preferably greater than about1.0 gram per hour per transducer and most preferably greater than about2.0 grams per hour per transducer.

The concentrations of soluble precursors in the liquid feed 102 willvary depending upon the particular materials involved and the particularparticle composition and particle morphology desired. For mostapplications, when soluble precursor(s) are used, the solubleprecursor(s) are present at a concentration of from about 1–50 weightpercent of the liquid feed. 102. In any event, however, when solubleprecursors are used, the precursors should be at a low enoughconcentration to permit the liquid feed to be ultrasonically atomizedand to prevent premature precipitation of materials from the liquid feed102. The concentration of suspended particulate precursors will alsovary depending upon the particular materials involved in the particularapplication.

One significant aspect of the process of the present invention formanufacturing particulate materials is the unique flow characteristicsencountered in the furnace relative to laboratory scale systems. Themaximum Reynolds number attained for flow in the furnace 110 with thepresent invention is very high, typically in excess of 500, preferablyin excess of 1,000 and more preferably in excess of 2,000. In mostinstances, however, the maximum Reynolds number for flow in the furnacewill not exceed 10,000, and preferably will not exceed 5,000. This issignificantly different from lab-scale systems where the Reynolds numberfor flow in a reactor is typically lower than 50 and rarely exceeds 100.

The Reynolds number is a dimensionless quantity characterizing flow of afluid which, for flow through a circular cross sectional conduit isdefined as:

${Re} = \frac{\rho\; v\; d}{\mu}$

where:

-   -   ρ=fluid density;    -   v=fluid mean velocity;    -   d=conduit inside diameter; and    -   μ=fluid viscosity.        It should be noted that the values for density, velocity and        viscosity will vary along the length of the furnace 110. The        maximum Reynolds number in the furnace 110 is typically attained        when the average stream temperature is at a maximum, because the        gas velocity is at a very high value due to gas expansion when        heated.

One problem with operating under flow conditions at a high Reynoldsnumber is that undesirable volatilization of components is much morelikely to occur than in systems having flow characteristics as found inlaboratory-scale systems. The volatilization problem occurs with thepresent invention, because the furnace is typically operated over asubstantial section of the heating zone in a constant wall heat fluxmode, due to limitations in heat transfer capability. This issignificantly different than operation of a furnace at a laboratoryscale, which typically involves operation of most of the heating zone ofthe furnace in a uniform wall temperature mode, because the heating loadis sufficiently small that the system is not heat transfer limited.

With the present invention, it is typically preferred to heat theaerosol stream in the heating zone of the furnace as quickly as possibleto the desired temperature range for particle manufacture. Because offlow characteristics in the furnace and heat transfer limitations,during rapid heating of the aerosol the wall temperature of the furnacecan significantly exceed the maximum average target temperature for thestream. This is a problem because, even though the average streamtemperature may be within the range desired, the wall temperature maybecome so hot that components in the vicinity of the wall are subjectedto temperatures high enough to undesirably volatilize the components.This volatilization near the wall of the furnace can cause formation ofsignificant quantities of ultrafine particles that are outside of thesize range desired.

Therefore, with the present invention, it is preferred that when theflow characteristics in the furnace are such that the Reynolds numberthrough any part of the furnace exceeds 500, more preferably exceeds1,000, and most preferably exceeds 2,000, the maximum wall temperaturein the furnace should be kept at a temperature that is below thetemperature at which a desired component of the final particles wouldexert a vapor pressure not exceeding about 200 millitorr, morepreferably not exceeding about 100 millitorr, and most preferably notexceeding about 50 millitorr. Furthermore, the maximum wall temperaturein the furnace should also be kept below a temperature at which anintermediate component, from which a final component is to be at leastpartially derived, should also have a vapor pressure not exceeding themagnitudes noted for components of the final product.

In addition to maintaining the furnace wall temperature below a levelthat could create volatilization problems, it is also important thatthis not be accomplished at the expense of the desired average streamtemperature. The maximum average stream temperature must be maintainedat a high enough level so that the particles will have a desired highdensity. The maximum average stream temperature should, however,generally be a temperature at which a component in the final particles,or an intermediate component from which a component in the finalparticles is at least partially derived, would exert a vapor pressurenot exceeding about 100 millitorr, preferably not exceeding about 50millitorr, and most preferably not exceeding about 25 millitorr.

So long as the maximum wall temperature and the average streamtemperature are kept below the point at which detrimental volatilizationoccurs, it is generally desirable to heat the stream as fast as possibleand to remove resulting particles from the furnace immediately after themaximum average stream temperature is reached in the furnace. With thepresent invention, the average residence time in the heating zone of thefurnace may typically be maintained at shorter than about 4 seconds,preferably shorter than about 2 seconds, more preferably shorter thanabout 1 second, still more preferably shorter than about 0.5 second, andmost preferably shorter than about 0.2 second.

Another significant issue with respect to operating the process of thepresent invention, which includes high aerosol flow rates, is losswithin the system of materials intended for incorporation into the finalparticulate product. Material losses in the system can be quite high ifthe system is not properly operated. If system losses are too high, theprocess would not be practical for use in the manufacture of particulateproducts of many materials. This has typically not been a majorconsideration with laboratory-scale systems.

One significant potential for loss with the process of the presentinvention is thermophoretic losses that occur when a hot aerosol streamis in the presence of a cooler surface. In that regard, the use of thequench cooler, as previously described, with the process of the presentinvention provides an efficient way to cool the particles withoutunreasonably high thermophoretic losses. There is also, however,significant potential for losses occurring near the end of the furnaceand between the furnace and the cooling unit.

It has been found that thermophoretic losses in the back end of thefurnace can be significantly controlled if the heating zone of thefurnace is operated such that the maximum average stream temperature isnot attained until near the end of the heating zone in the furnace, andat least not until the last third of the heating zone. When the heatingzone includes a plurality of heating sections, the maximum averagestream temperature should ordinarily not occur until at least the lastheating section. Furthermore, the heating zone should typically extendto as close to the exit of the furnace as possible. This is counter toconventional thought which is to typically maintain the exit portion ofthe furnace at a low temperature to avoid having to seal the furnaceoutlet at a high temperature. Such cooling of the exit portion of thefurnace, however, significantly promotes thermophoretic losses.Furthermore, the potential for operating problems that could result inthermophoretic losses at the back end of the furnace are reduced withthe very short residence times in the furnace for the present invention,as discussed previously.

Typically, it would be desirable to instantaneously cool the aerosolupon exiting the furnace. This is not possible. It is possible, however,to make the residence time between the furnace outlet and the coolingunit as short as possible. Furthermore, it is desirable to insulate theaerosol conduit occurring between the furnace exit and the cooling unitentrance. Even more preferred is to insulate that conduit and, even morepreferably, to also heat that conduit so that the wall temperature ofthat conduit is at least as high as the average stream temperature ofthe aerosol stream. Furthermore, it is desirable that the cooling unitoperate in a manner such that the aerosol is quickly cooled in a mannerto prevent thermophoretic losses during cooling. The quench cooler,described previously, is very effective for cooling with low losses.Furthermore, to keep the potential for thermophoretic losses very low,it is preferred that the residence time of the aerosol stream betweenattaining the maximum average stream temperature in the furnace and apoint at which the aerosol has been cooled to an average streamtemperature below about 200° C. is shorter than about 2 seconds, morepreferably shorter than about 1 second, and even more preferably shorterthan about 0.5 second and most preferably shorter than about 0.1 second.In most instances, the maximum average stream temperature attained inthe furnace will be greater than about 800° C. Furthermore, the totalresidence time from the beginning of the heating zone in the furnace toa point at which the average stream temperature is at a temperaturebelow about 200° C. should typically be shorter than about 5 seconds,preferably shorter than about 3 seconds, more preferably shorter thanabout 2 seconds, and most preferably shorter than about 1 second.

Another part of the process with significant potential forthermophoretic losses is after particle cooling until the particles arefinally collected. Proper particle collection is very important toreducing losses within the system. The potential for thermophoreticlosses is significant following particle cooling because the aerosolstream is still at an elevated temperature to prevent detrimentalcondensation of water in the aerosol stream. Therefore, cooler surfacesof particle collection equipment can result in significantthermophoretic losses.

To reduce the potential for thermophoretic losses before the particlesare finally collected, it is important that the transition between thecooling unit and particle collection be as short as possible.Preferably, the output from the quench cooler is immediately sent to aparticle separator, such as a filter unit or a cyclone. In that regard,the total residence time of the aerosol between attaining the maximumaverage stream temperature in the furnace and the final collection ofthe particles is preferably shorter than about 2 seconds, morepreferably shorter than about 1 second, still more preferably shorterthan about 0.5 second and most preferably shorter than about 0.1 second.Furthermore, the residence time between the beginning of the heatingzone in the furnace and final collection of the particles is preferablyshorter than about 6 seconds, more preferably shorter than about 3seconds, even more preferably shorter than about 2 seconds, and mostpreferably shorter than about 1 second. Furthermore, the potential forthermophoretic losses may further be reduced by insulating the conduitsection between the cooling unit and the particle collector and, evenmore preferably, by also insulating around the filter, when a filter isused for particle collection. The potential for losses may be reducedeven further by heating of the conduit section between the cooling unitand the particle collection equipment, so that the internal equipmentsurfaces are at least slightly warmer than the aerosol stream averagestream temperature. Furthermore, when a filter is used for particlecollection, the filter could be heated. For example, insulation could bewrapped around a filter unit, with electric heating inside of theinsulating layer to maintain the walls of the filter unit at a desiredelevated temperature higher than the temperature of filter elements inthe filter unit, thereby reducing thermophoretic particle losses towalls of the filter unit.

Even with careful operation to reduce thermophoretic losses, some losseswill still occur. For example, some particles will inevitably be lost towalls of particle collection equipment, such as the walls of a cycloneor filter housing. One way to reduce these losses, and correspondinglyincrease product yield, is to periodically wash the interior of theparticle collection equipment to remove particles adhering to the sides.In most cases, the wash fluid will be water, unless water would have adetrimental effect on one of the components of the particles. Forexample, the particle collection equipment could include parallelcollection paths. One path could be used for active particle collectionwhile the other is being washed. The wash could include an automatic ormanual flush without disconnecting the equipment. Alternatively, theequipment to be washed could be disconnected to permit access to theinterior of the equipment for a thorough wash. As an alternative tohaving parallel collection paths, the process could simply be shut downoccasionally to permit disconnection of the equipment for washing. Theremoved equipment could be replaced with a clean piece of equipment andthe process could then be resumed while the disconnected equipment isbeing washed.

For example, a cyclone or filter unit could periodically be disconnectedand particles adhering to interior walls could be removed by a waterwash. The particles could then be dried in a low temperature dryer,typically at a temperature of lower than about 50° C.

In one embodiment, wash fluid used to wash particles from the interiorwalls of particle collection equipment includes a surfactant. Some ofthe surfactant will adhere to the surface of the particles. This couldbe advantageous to reduce agglomeration tendency of the particles and toenhance dispersibility of the particles in a thick film pastformulation. The surfactant could be selected for compatibility with thespecific paste formulation anticipated.

Another area for potential losses in the system, and for the occurrenceof potential operating problems, is between the outlet of the aerosolgenerator and the inlet of the furnace. Losses here are not due tothermophoresis, but rather to liquid coming out of the aerosol andimpinging and collecting on conduit and equipment surfaces. Althoughthis loss is undesirable from a material yield standpoint, the loss maybe even more detrimental to other aspects of the process. For example,water collecting on surfaces may release large droplets that can lead tolarge particles that detrimentally contaminate the particulate product.Furthermore, if accumulated liquid reaches the furnace, the liquid cancause excessive temperature gradients within the furnace tube, which cancause furnace tube failure, especially for ceramic tubes. One way toreduce the potential for undesirable liquid buildup in the system is toprovide adequate drains, as previously described. In that regard, it ispreferred that a drain be placed as close as possible to the furnaceinlet to prevent liquid accumulations from reaching the furnace. Thedrain should be placed, however, far enough in advance of the furnaceinlet such that the stream temperature is lower than about 80° C. at thedrain location.

Another way to reduce the potential for undesirable liquid buildup isfor the conduit between the aerosol generator outlet and the furnaceinlet be of a substantially constant cross sectional area andconfiguration. Preferably, the conduit beginning with the aerosolgenerator outlet, passing through the furnace and continuing to at leastthe cooling unit inlet is of a substantially constant cross sectionalarea and geometry.

Another way to reduce the potential for undesirable buildup is to heatat least a portion, and preferably the entire length, of the conduitbetween the aerosol generator and the inlet to the furnace. For example,the conduit could be wrapped with a heating tape to maintain the insidewalls of the conduit at a temperature higher than the temperature of theaerosol. The aerosol would then tend to concentrate toward the center ofthe conduit due to thermophoresis. Fewer aerosol droplets would,therefore, be likely to impinge on conduit walls or other surfacesmaking the transition to the furnace.

Another way to reduce the potential for undesirable liquid buildup is tointroduce a dry gas into the aerosol between the aerosol generator andthe furnace. Referring now to FIG. 49, one embodiment of the process isshown for adding a dry gas 118 to the aerosol 108 before the furnace110. Addition of the dry gas 118 causes vaporization of at least a partof the moisture in the aerosol 108, and preferably substantially all ofthe moisture in the aerosol 108, to form a dried aerosol 119, which isthen introduced into the furnace 110.

The dry gas 118 will most often be dry air, although in some instancesit may be desirable to use dry nitrogen gas or some other dry gas. If asufficient quantity of the dry gas 118 is used, the droplets of theaerosol 108 are substantially completely dried to beneficially formdried precursor particles in aerosol form for introduction into thefurnace 110, where the precursor particles are then pyrolyzed to make adesired particulate product. Also, the use of the dry gas 118 typicallywill reduce the potential for contact between droplets of the aerosoland the conduit wall, especially in the critical area in the vicinity ofthe inlet to the furnace 110. In that regard, a preferred method forintroducing the dry gas 118 into the aerosol 108 is from a radialdirection into the aerosol 108. For example, equipment of substantiallythe same design as the quench cooler, described previously withreference to FIGS. 41–43, could be used, with the aerosol 108 flowingthrough the interior flow path of the apparatus and the dry gas 118being introduced through perforated wall of the perforated conduit. Analternative to using the dry gas 118 to dry the aerosol 108 would be touse a low temperature thermal preheater/dryer prior to the furnace 110to dry the aerosol 108 prior to introduction into the furnace 110. Thisalternative is not, however, preferred.

Still another way to reduce the potential for losses due to liquidaccumulation is to operate the process with equipment configurationssuch that the aerosol stream flows in a vertical direction from theaerosol generator to and through the furnace. For smaller-sizeparticles, those smaller than about 1.5 microns, this vertical flowshould, preferably, be vertically upward. For larger-size particles,such as those larger than about 1.5 microns, the vertical flow ispreferably vertically downward.

Furthermore, with the process of the present invention, the potentialfor system losses is significantly reduced because the total systemretention time from the outlet of the generator until collection of theparticles is typically shorter than about 10 seconds, preferably shorterthan about 7 seconds, more preferably shorter than about 5 seconds andmost preferably shorter than about 3 seconds.

Powders of a variety of materials may be made according to the presentinvention, with the powders so produced being an important aspect of theinvention.

With the present invention, these various powders may be made with verydesirable attributes for a variety of applications. In that regard, thepowders are typically made with a small weight average particle size,narrow particle size distribution, spheroidal particle shape, and highdensity relative to a theoretical density for the material of theparticles. Also, the particles of the powder typically are eithersubstantially single crystalline or are polycrystalline and with a largemean crystallite size.

With respect to particle size, the powders are characterized generallyas having a weight average particle size that typically is in the rangeof from about 0.05 micron to about 4 microns, with most powders having aweight average size of from about 0.1 micron to about 3 microns. Withthe process of the present invention, however, particle size maygenerally be controlled to provide particles with a desired size.Particle size is varied primarily by altering the frequency ofultrasonic transducers in the aerosol generator and by altering theconcentration of precursors in the liquid feed. Lower ultrasonicfrequencies tend to produce larger particles, while higher frequenciestend to produce smaller particles. Also, higher precursor concentrationsin the liquid feed tend to produce larger particles and lower precursorconcentrations in the liquid feed tend to produce smaller particles.

The particles are typically characterized as having a weight averageparticle size in a range having a lower limit, depending upon theapplication, of from about 0.1 micron, or about 0.2 micron, or about 0.3micron, or about 0.5 micron, or about 0.8 micron, or about 1 micron; andhaving an upper limit, depending upon the application, of about 4microns, or about 3 microns, or about 2.5 microns, or about 2 microns,or about 1 micron, or about 0.8 micron, or about 0.6 micron. Powdershaving a weight average size range defined by any combination of one ofthe specified upper limits and one of the specified lower limits arewithin the scope of the present invention, so long as the upper limit islarger than the lower limit. Some particularly preferred ranges forweight average particle size are provided below in discussions specificto certain material.

The powders are also characterized as having a narrow particle sizedistribution, typically with greater than about 75 weight percent,preferably greater than about 90 weight percent, and more preferablygreater than about 95 weight percent of the particles in the powderhaving a size of smaller than two times the weight average particlesize, and even more particularly smaller than about 1.5 times the weightaverage particle size.

The powders are also typically characterized as being comprised ofspheroidal particles. In that regard, the particles are substantiallyspherical, in that the particles are not jagged or irregular in shape,although the particles may become faceted as the crystallite size in theparticles increases. Spheroidal particles are advantageous because theytypically have increased dispersibility and flowability in pasteformulations relative to jagged or irregular particles.

Although in some instances the powders may be made as very porous orhollow particles, the powders are usually characterized as being verydense, with the particles typically having a density of at least about80%, preferably at least about 90% and more preferably at least about95%, of a theoretical density. The theoretical density is that densitythat particles would have assuming that the particles included zeroporosity. As used herein, the density of a particle is as measured byhelium pycnometry. High particle density is particularly advantageousfor thick film applications involving a fired film, because higherdensity particles tend to exhibit reduced shrinkage during sinteringthan highly porous particles.

The powders are further characterized as typically having a high degreeof purity, with generally no more than about 0.1 atomic percentimpurities and preferably no more than about 0.01 atomic percentimpurities. One significant characteristic of the powders of the presentinvention is that they may be made to be substantially free of organicmaterials, if desired, and particularly to be substantially free ofsurfactants. This is a significant advantage over particles made by aliquid route, which typically include residual surfactants. Theseresidual surfactants can significantly impair the utility of theparticles, especially in making thick film pastes.

As noted above, one group of powders of the present invention aremetal-containing powders. The metal in the particles may includevirtually any type of metal and can include both single-component metalsand metal alloys. Particularly preferred metal-containing powder batchesaccording to the present invention include at least one of palladium(Pd), silver (Ag), nickel (Ni), copper (Cu), gold (Au), platinum (Pt),molybdenum (Mo), tungsten (W), tantalum (Ta), aluminum (Al), and thelike. Preferred alloys can include a Ag/Pd alloy, such as one having aAg:Pd ratio of about 70:30.

Most preferred are metal-containing powders including at least one ofpalladium, silver, nickel, copper, gold and platinum, and even moreparticularly those including at least one of palladium, silver, nickeland copper.

The metal may be present in the particles in any convenient form, but istypically present in a metallic phase. The metallic phase may includesubstantially only a single metal or may include a metal alloy. Themetal alloy may include any desired relative amounts of alloyconstituents. When the powders include a metal alloy, the alloy istypically made by codissolving metal precursor salts in the liquid feedthat is aerosolized to make the powder. Also, when reference is makeherein to allows, it should be recognized that the discussion appliesequally to intermetallic compounds, which are not true alloys.

These metal-containing powders are primarily used for the manufacture ofelectrically conductive thick film features in electronic products.These thick film features are typically made by applying a layer of apaste containing the powder to a substrate, drying the layer to removevolatile components from layer, and firing the layer to form the film.Extremely high quality powders are required for many of thesemicroelectronic thick film applications and the presence of asignificant quantities of ultrafine particles should be avoided.Therefore, when making the metal-containing powders of the presentinvention, care should be exercised during powder manufacture so thatthe vapor pressure of the metal components does not reach a detrimentallevel, as previously discussed.

Because of the relative importance of powders including at least one ofpalladium, silver, nickel, copper, gold and platinum, those particularmetals will be discussed in greater detail.

Palladium-containing powders of the present invention are useful inmaking electrically conductive features for a variety of microelectronicdevices. The powders may be used, for example, to make internalelectrodes for multi-layer capacitors, conductive lines and otherconductive pathways in multi-chip modules, and address electrodes forflat panel displays. One use for the palladium-containing particles isfor making metallized terminations for multi-layer ceramic capacitorsand other microelectronic devices.

Palladium precursors are typically palladium salts. Preferred precursorsare nitrate salts. The palladium is typically present in the powders ina metallic phase, which may include substantially only palladium or mayinclude the palladium in an alloy with one or more other metals.Alloying elements include, but are not limited to, silver (Ag), nickel(Ni), copper (Cu), platinum (Pt), molybdenum (Mo), tungsten (W),tantalum (Ta), aluminum (Al), gold (Au), indium (In), lead (Pb), tin(Sn), bismuth (Bi) and the like. Particularly preferred for alloyingwith palladium are silver and nickel, and particularly silver. Thealloying element is typically present in the alloy in an amount of fromabout 0.1 to about 40 weight percent, with from about 1 to about 30weight percent being more preferred, based on the total weight of thealloy, with the balance of the alloy typically comprising palladium.

The palladium-containing powders may have any convenient weight averageparticle size within the range of the invention, which will varydepending upon the application. For most applications, the weightaverage particle size will be in a range of from about 0.1 micron toabout 2 microns. When used to make electrically conductive features inmicroelectronic devices, such as multi-layer ceramic capacitors ormulti-chip modules, the powder preferably has a weight average particlesize of from about 0.1 micron to about 0.8 micron. For use in makingelectrodes for flat panel displays, the particles preferably have aweight average particle size of from about 1 micron to about 3 microns.A preferred average particle size for metallized terminations formulti-layer ceramic capacitors and other microelectronic devices is fromabout 1 micron to about 3 microns.

Silver-containing powders of the present invention are useful in makingelectrically conductive features for a variety of microelectronicdevices. The powders may be used, for example, to make internalelectrodes for multi-layer capacitors, conductive lines and otherconductive pathways in multi-chip modules, and address electrodes forflat panel displays. One significant use for the silver-containingparticles is for making metallized terminations for multi-layer ceramiccapacitors and other microelectronic devices. Another significant use isfor use as particulate electrode materials, such as or electrochemicalcells, including zinc-air cells.

Silver precursors are typically silver salts, with nitrate salts beingpreferred. The silver is typically present in the powders in a metallicphase, which may include substantially only silver or may include thesilver in an alloy with one or more other metals. Alloying elementsinclude, but are not limited to, palladium (Pd), nickel (Ni), copper(Cu), platinum (Pt), molybdenum (Mo), tungsten (W), tantalum (Ta),aluminum (Al), gold (Au), indium (In), lead (Pb), tin (Sn), bismuth (Bi)and the like. Particularly preferred for alloying with silver arepalladium and platinum. The alloying element is typically present in thealloy in an amount of from about 0.1 to about 40 weight percent, withfrom about 1 to about 30 weight percent being more preferred, based onthe total weight of the alloy, with the balance of the alloy typicallycomprising silver.

The silver-containing powders may have any convenient weight averageparticle size within the range of the invention, which will varydepending upon the application. For most applications, the weightaverage particle size will be in a range of from about 0.1 micron toabout 2 microns. When used to make electrically conductive features inmicroelectronic devices, such as multi-layer ceramic capacitors ormulti-chip modules, the powder preferably has a weight average particlesize of from about 0.1 micron to about 0.8 micron. For use in makingelectrodes for flat panel displays, the particles preferably have aweight average particle size of from about 1 micron to about 3 microns.A preferred average particle size for metallized terminations formulti-layer ceramic capacitors and other microelectronic devices is fromabout 1 micron to about 3 microns.

Nickel-containing powders of the present invention are useful in makingelectrically conductive features for a variety of microelectronicdevices. The powders may be used, for example, to make internalelectrodes for multi-layer capacitors, conductive lines and otherconductive pathways in multi-chip modules, and address electrodes forflat panel displays.

Precursors for making nickel-containing powders are typically nickelsalts that are soluble in water, with nitrate salts being preferred. Thenickel-containing powders may include the nickel in a metallic phase ora nonmetallic phase, such as in the form of nickel boride or nickeloxide. Most often, however, the nickel is in a metallic form.

When the nickel is present in a metallic phase, it may be in a phase ofsubstantially only nickel, or it may be in the form of an alloy with oneor more other metals. Alloying elements include, but are not limited to,palladium (Pd), silver (Ag), gold (Au), copper (Cu), tungsten (W),molybdenum (Mo), platinum (Pt), iron (Fe) and cobalt (Co). In onepreferred embodiment, the alloying element is palladium. The alloyingelement is typically present in the alloy in an amount of from about 0.1to about 40 weight percent, with from about 1 to about 15 weight percentbeing more preferred, based on the total weight of the alloy, with thebalance of the alloy typically comprising nickel.

The nickel-containing powders may have any convenient weight averageparticle size within the range of the invention, which will varydepending upon the application. For most applications, the weightaverage particle size will be in a range of from about 0.1 micron toabout 2 microns. When used to make electrically conductive features inmicroelectronic devices, such as multi-layer ceramic capacitors ormulti-chip modules, the powder preferably has a weight average particlesize of from about 0.1 micron to about 0.8 micron. For use in makingelectrodes for flat panel displays, the particles preferably have aweight average particle size of from about 1 micron to about 3 microns.

Copper-containing powders of the present invention are useful in makingelectrically conductive features for a variety of microelectronicdevices. The powders may be used, for example, to make internalelectrodes for multi-layer capacitors, conductive lines and otherconductive pathways in multi-chip modules, and address electrodes forflat panel displays. One significant use for the copper-containingparticles is for making metallized terminations for multi-layer ceramiccapacitors and other microelectronic devices.

Precursors for making copper-containing powders are typically coppersalts that are soluble in water, with nitrate salts being preferred. Thecopper is typically present in the powders in a metallic phase, whichmay include substantially only copper or may include the copper in analloy with one or more other metals. Alloying elements include, but arenot limited to, palladium (Pd), silver (Ag), gold (Au), nickel (Ni),tungsten (W), molybdenum (Mo), aluminum (Al), zinc (Zn), magnesium (Mg),tin (Sn), beryllium (Be) and platinum (Pt). Zinc is a particularlypreferred alloying element for increasing the oxidation resistance ofthe copper metal. The alloying element is typically present in the alloyin an amount of from about 0.1 to about 40 weight percent, with fromabout 1 to about 15 weight percent being more preferred, based on thetotal weight of the alloy, with the balance of the alloy typicallycomprising copper.

The copper-containing powders may have any convenient weight averageparticle size within the range of the invention, which will varydepending upon the application. For most applications, the weightaverage particle size will be in a range of from about 0.1 micron toabout 2 microns. When used to make electrically conductive features inmicroelectronic devices, such as multi-layer ceramic capacitors ormulti-chip modules, the powder preferably has a weight average particlesize of from about 0.1 micron to about 0.8 micron. For use in makingelectrodes for flat panel displays, the particles preferably have aweight average particle size of from about 1 micron to about 3 microns.A preferred average particle size for metallized terminations formulti-layer ceramic capacitors and other microelectronic devices is fromabout 1 micron to about 3 microns.

Gold-containing powders of the present invention are useful in makingelectrically conductive features for a variety of microelectronicdevices.

Precursors used for the gold in gold-containing powders are typicallywater-soluble gold salts, with chloride salts being preferred. The goldis typically present in the powders in a metallic phase, which mayinclude substantially only gold or may include the gold in an alloy withone or more other metals. Alloying elements include, but are not limitedto, palladium (Pd), silver (Ag), nickel (Ni), tungsten (W), molybdenum(Mo) and platinum (Pt). Particularly preferred are alloys with platinumor palladium. The alloying element is typically present in the alloy inan amount of from about 0.1 to about 40 weight percent, with from about1 to about 15 weight percent being more preferred, based on the totalweight of the alloy, with the balance of the alloy typically comprisinggold.

The gold-containing powders may have any convenient weight averageparticle size within the range of the invention, which will varydepending upon the application. For most applications, the weightaverage particle size will be in a range of from about 0.05 micron toabout 2 microns. When used to make electrically conductive features inmicroelectronic devices, the powder preferably has a weight averageparticle size of from about 0.1 micron to about 1 micron.

Platinum-containing powders of the present invention are useful inmaking electrically conductive features for a variety of microelectronicdevices.

Precursors for the platinum in the platinum-containing powders aretypically water-soluble platinum compound. One preferred precursor ischoroplatinic acid. The platinum is typically present in the powders ina metallic phase, which may include substantially only platinum or mayinclude the platinum in an alloy with one or more other metals. Alloyingelements include, but are not limited to, palladium (Pd), silver (Ag),nickel (Ni), copper (Cu), tungsten (W), molybdenum (Mo) and gold (Au).Particularly preferred are alloys with gold or palladium. The alloyingelement is typically present in the alloy in an amount of from about 0.1to about 40 weight percent, with from about 1 to about 15 weight percentbeing more preferred, based on the total weight of the alloy, with thebalance of the alloy typically comprising platinum.

The platinum-containing powders may have any convenient weight averageparticle size within the range of the invention, which will varydepending upon the application. For most applications, the weightaverage particle size will be in a range of from about 0.05 micron toabout 2 microns. When used to make electrically conductive features inmicroelectronic devices, the powder preferably has a weight averageparticle size of from about 0.1 micron to about 1 micron.

The metal-containing particles of the present invention may include onlya single material phase, which would include the noted metal.Alternatively, the metal-containing particles may be multi-phase, orcomposite, particles. In multi-phase particles, the metal is present ina first material phase. The particles also include a second materialphase that is different than the first material phase. The multi-phaseparticles may, however, include more than two material phases.

Single phase particles will typically consist essentially of a singlemetallic phase of the metal or an alloy including the metal. Multi-phaseparticles also typically include a metallic phase including the metaland also include at least one other phase. Besides the metal-containingmetallic phase, the other phases that may be present are other metallicphases, that are preferably substantially free of the metal, ornonmetallic phases, that are also preferably substantially free of themetal.

For many applications, whether single phase or multi-phase particles areused, the metal-containing metallic phase will frequently comprisegreater than about 50 weight percent of the metal, preferably greaterthan about 60 weight percent of the metal, more preferably greater thanabout 70 weight percent of the metal, even more preferably greater thanabout 80 weight percent of the metal and most preferably greater thanabout 90 weight percent of the metal.

Multi-phase particles may be desirable for a number of reasons,including: (1) a reduction in the amount of the an expensive metal thatis used in the particle to provide electrical conductivity byincorporating a second material phase that is a less expensive fillermaterial; (2) to improve flowability of the particles in a paste and toimprove resistance of particles to deformations; (3) to modify physicalproperties of the particles for improved compatibility with a substratesupporting a conductive film made using the particles, includingmodifications of the thermal coefficient of linear expansion,modification of sintering/densification characteristics, andmodification of surface energy to alter wetability of the particles; and(4) to modify electrical or dielectric properties for customizedelectronic components. Some examples of uses of the multi-phase,metal-containing particles include use as catalysts or catalyticsupports and as particles in paste formulations used in thick filmapplications, including manufacture of multi-layer capacitors,multi-chip components, super capacitors and other electronic components,batteries and fuel cells.

A significant aspect of the present invention is the extremely highquality, metal-containing, multi-phase particles, preferably includingat least one of palladium, silver, nickel, copper, gold and platinum(and especially including at least one of palladium, silver, nickel andcopper), that may be made according to the process previously described.These multi-phase powders include multi-phase particles having at leasta first material phase and a second material phase. Additional materialphases may be present, if desired. The first material phase includes themetal, and is typically an electrically conductive metallic phase, withthe metal being in the form of the substantially pure metal or an alloywith one or more other metal. The second material phase, which isdifferent than the first material phase, is typically substantially freeof the metal.

The second material phase may be a metallic phase. When the secondmaterial phase is a metallic phase, it may be a substantially puresingle metal, or may include an alloy of two or more metals. When one ofpalladium, silver, nickel, copper, gold and platinum is in the firstmaterial phase, one or more of the remaining of those metals may bepresent in the second material phase. Examples of some other metals thatmay be included in the second material phase include molybdenum,tungsten, tantalum, aluminum, indium, lead, tin, bismuth, and the like.

For most applications, however, the second material phase will benonmetallic, in which case the second material phase will also typicallynot be electrically conductive. Preferred in a nonmetallic secondmaterial phase are a variety of ceramic materials, glass materials orother materials that would alter the sintering and/or densificationcharacteristics of the particles. Control of sintering and/ordensification characteristics of the particles is particularly importantwhen the particles are to be used in a thick film paste for manufactureof a metal-containing film on a substrate including a ceramic layer,which is typically dielectric, to more closely match with the sinteringand shrinkage characteristics of the powder particles with those of thesubstrate, thereby reducing the occurrence of problems such as filmcracking and delamination. This is particularly important when layersare to be cofired, such as in multi-layer ceramic capacitors andmulti-chip modules.

The second material phase may include an oxide material, such as oxidesof zinc, tin, barium, molybdenum, manganese, vanadium, niobium,tantalum, tungsten, iron, silver, chromium, cobalt, nickel, copper,yttrium, iridium, beryllium, silicon, zirconium, aluminum, bismuth,magnesium, thorium and gadolinium. Some preferred oxides are silica,alumina, titania, zirconia, yttria, and oxides of copper, bismuth andtin. Another preferred group of oxides includes borates, titanates,silicates (including borosilicates and aluminosilicates), aluminates,niobates, zirconates and tantalates. Examples include mullite,cordierite, barium titanate, neodymium titanate, magnesium titanate,calcium titanate, strontium titanate and lead titanate. Additionalmaterials that could be used as the second material phase include glassmaterials, such as glass frits and glazes. Particularly preferred aresecond material phases including titanates, and especially including atitanate of one or more of barium, strontium, neodymium, calcium,magnesium and lead. The titanate may be of a single metal or may be amixed metal titanate, such as, for example Ba_(x)Sr_(1-x)TiO₃.Furthermore, a variety of other ceramic materials may be used in thesecond material phase, such as carbides, borides and nitrides, includingsilicon nitride. Also, porcelain could be used in the second materialphase.

The multi-phase particles of the present invention may typically be usedin place of single phase metallic particles for most application, solong as the proportion of the second material phase making up theparticles is small enough not to be detrimental to the application.Often, however, the use of multi-phase particles significantly enhancesthe performance of films made using the particles relative to the use ofsingle phase metallic particles.

One use for the multi-phase particles of the present invention is toform a film including the metal in a metallic phase, often electricallyconductive, adjacent to a layer of nonmetallic material, oftendielectric. In that case, the multi-phase particles will typicallyinclude in the second phase a nonmetallic material that enhancessuitability for use with the nonmetallic layer, resulting in improvedcompatibility and bonding between the nonmetallic layer and theelectrically conductive film including the metallic phase. For many ofthese applications, the multi-phase metal-containing particles willinclude in the second material phase a nonmetallic material that is alsopresent in an adjacent nonmetallic layer. Thus, when the nonmetalliclayer is of a dielectric material, that dielectric material is alsopresent in the second material phase. When the nonmetallic layer is aceramic layer, for example, the multi-phase particles could include inthe second phase a ceramic material that is also present in the ceramiclayer. As one specific example, titanate materials are often used in thedielectric layers of multi-layer capacitors, and the metal-containingparticles used to make internal electrodes for the multi-layer capacitorcould include in the second material phase the same titanate that ispresent in the dielectric layers. Electronic devices made using themulti-phase particles of the present invention, and especiallymulti-layer ceramic capacitors having internal electrode layers madeusing the particles, and the methods for making such devices are withinthe scope of the present invention.

Generally, for applications involving the use of multi-phase particlesto form a metallic electrically conductive phase adjacent a dielectriclayer, the second material phase of the particles typically comprisesless than about 30 weight percent of the particles, preferably less thanabout 20 weight percent of the particles, and more preferably less thanabout 10 weight percent of the particles.

Multiphase particles having a very low content of the second materialphase are generally preferred when the particles will be used to makeelectrically conductive features, because the second material phase istypically dielectric and reduces electrical conductivity. In manyinstances, therefore, and especially those including silica, alumina ora titanate as the second material phase, the second material phasetypically comprises less than about 10 weight percent of the particles,more preferably less than about 5 weight percent of the particles, andeven more preferably less than about 2 weight percent of the particles;but the second material phase will typically be at least about 0.1weight percent, and preferably at least about 0.5 weight percent, of theparticles. In this way, enhanced compatibility between the dielectriclayer and the electrically conductive film may be accomplished withoutsignificant detrimental impact to electrical conductivity. Also, the useof the multiphase particles to make electrically conductive films willtypically result in improved adhesion for better bonding with thedielectric layer, thereby reducing the potential for delaminations.

One particularly preferred powder of multi-phase particles includes ametallic first material phase, which is preferably electricallyconductive, and a second material phase including a ceramic material ofat least one of silica, alumina, zirconia and titania, with the secondmaterial phase preferably being dielectric. Preferred as the firstmaterial phase is a metallic phase including at least one of palladium,silver, nickel, copper, gold and platinum. Especially preferred in thefirst material phase is at least one of palladium, silver, nickel andcopper. When the second material phase comprises silica or alumina, thepowder will typically include the first material phase as thepredominant phase, especially when the particles are designed for use tomake electrically conductive thick film features. When the secondmaterial phase includes zirconia or titania, however, the secondmaterial phase may be the predominant phase, especially when theparticles are designed for use as catalysts.

Another particularly preferred powder of the present invention includingmulti-phase particles includes an electrically conductive metallic firstmaterial phase, typically as the predominant phase, and a secondmaterial phase including a titanate. Preferably, the titanate is of oneor more of barium, strontium, neodymium, calcium, magnesium and lead.The first material phase preferably includes one or more of palladium,silver, nickel, copper, gold and platinum, and especially one or more ofpalladium, silver, nickel and copper.

Yet another particularly preferred powder of multi-phase particlesincludes the metallic first phase and a second phase including carbon.The carbon is typically an electrically conductive form of carbon, suchas in the form of graphite or carbon black. The metallic phasepreferably includes one or more of palladium, silver, nickel, copper,gold and platinum. The particles typically include the second materialphase as the predominant phase. The first material phase typically isused as a catalyst. These multi-phase particles are particularly wellsuited for use as catalysts and, depending upon the metal, electrodematerials, and especially as electrode materials in electrochemicalcells. These multi-phase powders may also be advantageously used asconductive filler particles in electrically conductive adhesiveformulations. One preferred powder includes silver as the metallicphase, especially for use as a cathode material for zinc-air batteriesor for fuel cells. Another preferred powder includes platinum,especially for use as cathode material for fuel cells. For mostapplications, the powder less than about 20 weight percent of the firstmaterial phase and preferably less than about 10 weight percent of thefirst material phase. The particles will, however, typically include atleast about 1 weight percent of the first material phase, preferably atleast 2 weight percent of the first material phase and more preferablyat least about 5 weight percent of the first material phase.Particularly preferred is for the powder to include from about 5 weightpercent to about 10 weight percent of the first material phase, andespecially about 7 weight percent of the first material phase.

A number of different variations of the process of the present inventionare possible for making the multi-phase particles. In one embodiment, ametal-containing precursor for the first material phase and a secondprecursor for the second material phase may both be included in theliquid feed 102 (referring back to FIGS. 1–49 and the discussionrelating thereto). In such a case, both precursors could be in solutionin a flowable liquid of the liquid feed 102. Alternatively, one or bothof the precursors could be particles suspended in the flowable liquid.Also, it is possible that the liquid feed 102 could include more thantwo precursors for the multi-phase particles. In another embodiment, themetal-containing precursor could initially be in the liquid feed 102,which is then processed in aerosol form in the furnace 110 to preparemetal-containing precursor particles. The precursor particles are thencoated with the second material phase in a separate step, in a mannersimilar to that described previously with reference to FIG. 45. Thistwo-step process of initially preparing metal-containing precursorparticles and then coating the precursor particles on the fly in anaerosol state is particularly advantageous because problems are avoidedthat are encountered in particle manufacture procedures, such as liquidroute precipitation, in which precursor particles would have to becollected and then redispersed prior to coating. Not only is collectionand redispersion cumbersome, but problems are often encountered due toparticle agglomeration, which is avoided with the on-the-fly coating ofthe present invention. Avoidance of particle agglomeration is veryimportant when a uniform particle coating is desired.

As noted previously, the multi-phase particles of the present inventionmay include a variety of particle morphologies. With reference again toFIG. 47, the multi-phase particles may include an intimate mixture ofthe first material phase and the second material phase, as in themulti-phase particle 500. Typically, with such an intimate mixture, thefirst material phase is a continuous phase throughout which the secondmaterial phase is dispersed. Another possible morphology is for thefirst material phase to be in the form of a large core that is coveredby a thin coating layer of the second material phase, as shown forparticles 502 and 504 in FIG. 47. Whether such coatings form a smoothcoating, such as shown in particle 502, or a rough and bumpy coating,such as shown in particle 504, will depend upon the wetabilitycharacteristics of the first and second material phases and theconditions under which the materials are processed, and especially theprocessing temperature. For example, in gas-to-particle conversionprocesses, higher temperatures during the coating operation tends toresult in smoother, more continuous coatings. The multi-phase particlescould also include a small core of one material phase surrounded by athick layer of the other material phase, as shown for particle 506.Also, the first and second material phase could completely segregate ina manner shown for particle 508. Furthermore, the multi-phase particlesare not limited to two material phases. For example, particle 510 inFIG. 47 shows a multi-phase particle including a core of second materialphase domains dispersed in a matrix of the first material phase, andwith the core being coated by a third material phase.

With continued reference to FIG. 47, it should be noted that the firstmaterial phase and the second material phase could constitute any of thephases in particles 500, 502, 504, 506, 508 and 510. For mostapplications, however, the first material phase, which includes themetal, will be the more abundant phase, and the second material phasewill be the less abundant phase.

In the case of coated particles, the second material phase will often bein the form of a coating around a core including the first materialphase. In the case of catalyst materials, however, the first materialphase may be a coating or a disperse phase on the surface of a supportof the second material phase. For particles including intimate mixturesof the two phases, the first material phase will typically be thecontinuous phase and the second material phase will typically be thedisperse phase.

For most applications, the multi-phase particles will include greaterthan about 50 weight percent of the first material phase, morepreferably greater than about 60 weight percent of the first materialphase, even more preferably greater than about 70 weight percent of thefirst material phase and most preferably greater than about 80 weightpercent of the first material phase. In the case of multi-phaseparticles including thin coating layers of the second material phase,the first material phase may comprise 90 weight percent or more of theparticles. Conversely, the second material phase typically will compriseless than about 50 weight percent of the multi-phase particles,preferably less than about 40 weight percent, more preferably less thanabout 30 weight percent and even more preferably less than about 20weight percent. In the case of thin coatings of the second materialphase, the second material phase may comprise 10 percent or less of theparticles. Even in the case of coated particles, however, the secondmaterial phase will typically comprise greater than about 0.5 weightpercent, and preferably greater than about 1 weight percent, of theparticles.

Because most applications for multi-phase particles of the presentinvention include the use of either a particle including the firstmaterial phase in a large core surrounded by a thin coating of thesecond material phase or an intimate mixture of the first material phaseas a continuous phase with the second material as a disperse phase,those particular situations will now be discussed below in greaterdetail.

One preferred class of multi-phase particles are coated particles inwhich the first material phase forms a core and the second materialphase forms a thin coating layer about the outer surface of the core.The second material phase may include any of the materials previouslylisted as being suitable for the second material phase.

With the present invention, the coating including the second materialphase may be made as a relatively uniform layer that substantiallyentirely covers the core of the first material phase. One method formaking multi-phase particles including the second material phase as auniform coating is as described previously with reference to FIG. 45. Inthat regard, the second material phase is typically formed on aprecursor particle, which includes the first material phase, bytechniques as previously described. A preferred coating technique isCVD. CVD is a well known deposition technique in which a precursor forthe second material phase is reacted in the vapor phase to form thesecond material phase. Generally, precursors for CVD aremetal-containing compounds, for example, inorganic compounds, metalorganics and organometallics. Examples of some vapor phase precursorsfor CVD of inorganic coatings include silanes, metal formates, metalacetates, metal oxalates, metal carboxylates, metal alkyls, metal aryls,metal alkoxides, metal ketonates (especially beta-diketonates), metalamides, metal hydrides, metal oxyhalides, and metal halides (especiallymetal chlorides and metal bromides). For example, to deposit a coatingof silica, a vaporous silane precursor, such as silicon tetrachloride,may be decomposed and converted to silica at elevated temperature in thepresence of oxygen or water vapor, with the silica then depositing onthe surface of metal-containing precursor particles.

Typically, a coating deposited by CVD or by PVD will result in anaverage coating thickness of from about 10 nanometers to about 200nanometers. Preferred coatings have an average thickness of thinner thanabout 100 nanometers, more preferably thinner than about 50 nanometersand most preferably thinner than about 25 nanometers.

Applications for coated multi-phase particles include the manufacture ofelectrically conductive films for electronic devices, such asmulti-layer capacitors and multi-chip modules. In the case of manycoatings such as silica, the coating is useful to beneficially alter thesintering and/or shrinkage characteristics of the particle for improvedcompatibility with a ceramic substrate.

Another way to make coated multi-phase particles is to provideprecursors for both the first material phase and the second materialphase in the feed liquid 102 (as described with respect to FIGS. 1–49).As noted previously, each precursor in the feed liquid 102 could beeither in the liquid phase, e.g., in solution in a flowable liquid, orin the form of particles suspended by the flowable liquid. Themulti-phase particles would then form in the furnace 110 as liquid isremoved from aerosol droplets. It should be noted that, in the case ofmultiple phases forming simultaneously in the furnace, the differentphases are typically initially formed as an intimate mixture of thephases. Generally, higher processing temperatures and longer residencetimes will result in redistribution of the material phases to thedesired morphology of a coating of one material phase about a core ofthe other material phase, assuming that the two material phases have theproper wetability characteristics. Alternatively, it is possible thatredistribution of the phases could result in complete segregation of thephases, as shown by the multi-phase particles 508 in FIG. 47. Whenredistribution of the material phases is desired to form a coatedparticle morphology, a processing embodiment such as that describedpreviously with reference to FIG. 46 may be advantageous.

When making coated particles with precursors for both the first materialphase and the second material phase in the liquid feed 102, a firstprecursor for the metal-containing first material phase could comprisepreformed metal-containing particles to be coated. The precursor for thesecond material phase could also be in particulate form, or could be insolution in a liquid phase. For example, a soluble precursor, such asfrom dissolution of a metal alkoxide could be used as a precursor forthe second material phase. In the case of metal alkoxides, it should berecognized that in aqueous solution the dissolved metal alkoxide usuallyreacts to form other soluble components, which will function as asoluble precursor. This could be the case in the preparation ofparticles including titania or alumina as the second material phase. Inthe case of silica as the second material phase, the precursor willtypically be small silica particles, which are preferably of colloidalsize, or silica dissolved in solution.

The manufacture of multi-phase particles with an intimately mixedmorphology for the different material phases is typically accomplishedby initially including a precursor for both the first material phase andthe second material phase in a liquid feed 102, as previously described.As noted, the process may be substantially the same as the process usedto prepare particles with a coating morphology, except the processingconditions may be altered so that the material phases do notredistribute, and are instead retained in an intimately mixed state.Generally, lower operating temperatures in the furnace 110 and shorterresidence times, with rapid particle cooling, promote an intimatemixture of the phases.

Multi-phase particles of an intimately mixed morphology are particularlyuseful for modifying sintering/densification temperatures of theparticle, reducing shrinkage that occurs during firing in thick filmapplications, and modifying the electrical or other properties of theparticle for special applications.

Another preferred group of powders of the present invention includephosphor particles. Phosphors are materials which are capable ofemitting radiation in the visible or ultraviolet spectral range uponexcitation, such as excitation by an external electric field or otherexternal energy source. Phosphors include a matrix compound, referred toas a host material, and the phosphor typically further includes one ormore dopants, referred to as activator ions, to emit a specific color orto enhance the luminescence characteristics.

Particular phosphor compounds may be preferred for certain applicationsand no single phosphor compound is necessarily preferred for allpossible applications. However, preferred phosphor host materials forsome display applications include the Group II sulfides (e.g., CaS, SrS,BaS, MgS, Mg_(x)Sr_(1-x)S and Ca_(x)Sr_(1-x)S) and the Group XIIsulfides (e.g., ZnS, CdS and Zn_(x)Cd_(1-x)S). Among these, ZnS isparticularly preferred for many display applications, particularly thoseutilizing high voltages (i.e. greater than about 2000 volts), dueprimarily to the high brightness of ZnS. ZnS is typically doped with Cu,Ag, Al, Au, Cl or mixtures thereof. For example, ZnS:Ag⁺¹ is a commonphosphor used to produce blue light in a CRT device.

Among the oxides, Y₂O₃ doped with Eu³⁺ (Y₂O₃:Eu³⁺) is often preferredfor emitting red light. BaMgAl₁₁O₁₇:Eu²⁺ (BAM:Eu²⁺) is also a commonoxide for producing red light. Other compounds that would be useful ifavailable include SrGa₂S₄:Eu²⁺, SrGa₂S₄:Ce³⁺, CaGa₂S₄:Eu²⁺ andCaGa₂S₄:Ce³⁺. Preferred phosphor host compounds and activators forparticular applications are discussed in more detail hereinbelow.

TABLE I Examples of Phosphor Materials Host Material Activator Ion ColorBaS Ce Yellow CaS Ce Green CaS Mn Yellow SrS Ce Blue-GreenMg_(x)Sr_(1−x)S Ce Blue-Green ZnS Cu Blue-Green BAM Eu Blue ZnO Zn GreenY₂O₃ Eu Red (Ce, Gd)MgB₅O₁₀ Tb Green Y₂O₂S Eu Red

The powder characteristics that are preferred will depend upon theapplication of the phosphor powders. Nonetheless, it can be generallystated that the powders should usually have a small particle size,narrow size distribution, spherical morphology, high density/lowporosity, high crystallinity and homogenous dopant distribution. Theefficiency of the phosphor, defined as the overall conversion ofexcitation energy to visible photons, should be high.

For most phosphor applications, the average particle size is morepreferably from about 0.1 micron to about 4 microns and even morepreferably is from about 0.5 micron to about 2 microns. The phosphorparticles producible according to the present invention can besubstantially single crystal particles or may be comprised of a numberof crystallites. It is possible according to the present invention toproduce phosphor particles having large crystallites. Crystallite sizecan be determined from the width of the x-ray diffraction peaks of thematerial. Large crystallites give rise to sharp peaks, while the peakwidth increases as crystallite size decreases.

It is preferred that the average crystallite size within the particlesis at least about 50 nanometers and more preferably is at least about100 nanometers. The average crystallite size most preferably approachesthe average particle size such that the particles are mostly singlecrystals. Preferably, the average crystallite size is at least about 50percent and more preferably at least about 80 percent of the averageparticle size. Highly crystalline phosphors (i.e. large crystallitesize) are believed to have increased efficiency as compared to phosphorswith smaller crystallites.

The phosphor particles producible according to the present inventionadvantageously have a high degree of purity, that is, a low level ofimpurities. Impurities are those materials that are not intended in thefinal product—thus, an activator ion is not considered an impurity. Thelevel of impurities in the phosphor powders of the present invention ispreferably less than about 0.1 weight percent and is more preferablyless than about 0.01 weight percent. Further, the activator ion ishomogeneously dispersed throughout the host material.

The particles of the present invention are also substantially sphericalin shape. Spherical particles are particularly advantageous because theyare able to disperse and coat a device more uniformly. As a result, thephosphor powder batch of the present invention is substantiallynon-agglomerated and has good dispersibility in a variety of media.

It is often advantageous to provide phosphor particles with a coating onthe outer surface thereof. Coatings are often desirable to reducedegradation of the phosphor material due to moisture or other influencessuch as the plasma in a plasma display device or electron bombardment incathodoluminescent devices. For example, metal sulfides such as ZnS areparticularly susceptible to degradation due to moisture and should becompletely encapsulated to reduce or eliminate the degradation reaction.Other phosphors are known to degrade in an electron beam operating at ahigh current density, such as in field emission displays and CRT's.

Preferred coatings include metal oxides such as SiO₂, MgO, Al₂O₃, SnO₂or In₂O₃. Semiconductive oxide coatings such as SnO or In₂O₃ canadvantageously absorb secondary electrons that are often emitted by thephosphor. The coatings can be either particulate coatings ornon-particulate (film) coatings. The coatings should be relatively thinand uniform. Preferably, the coating has an average thickness of lessthan about 1 micron and more preferably the average coating thickness isfrom about 5 nanometers to about 100 nanometers. Further, the particlescan include more than one coating substantially encapsulating theparticles to achieve the desired properties.

In addition, the phosphor particles can include organic coatings such asPMMA (polymethylmethacrylate), polystyrene or the like. The organiccoating should be on the order of 1 to 100 nanometers thick and besubstantially dense and continuous about the particle. Such coatings canbe formed after the powders are prepared by a liquid phase process. Theorganic coatings can advantageously prevent corrosion of the phosphorparticles especially in electroluminiscent lamps and also can improvethe dispersion characteristics of the particles.

The coating can also be a monolayer coating formed by the reaction of anorganic or an inorganic molecule with the surface of the phosphorparticles to form a coating that is essentially one molecular layerthick. In particular, the formation of a monolayer coating by reactionof the surface of the phosphor powder with a functionalized organosilane such as halo- or amino-silanes, for example hexamethyldisilazaneor trimethylsilylchloride, can be used to modify/control thehydrophobicity and hydrophilicity of the phosphor powders. Such coatingsallow for greater control over the dispersion characteristics of thephosphor powder in a wide variety of paste compositions.

The monolayer coatings may also be applied to phosphor powders that havealready been coated with, for example, 1–100 nanometer organic orinorganic coatings thus providing better control over the corrosioncharacteristics (through the use of thicker coating) as well asdispersibility (through the monolayer coating) of the phosphor powder.

More specifically, doped metal sulfide phosphors (MS:M′) can be preparedfrom an aqueous solution by the reaction of a metal carbonate (or oxideor hydroxide) with thiourea or a sulfur-containing acid such asthioacetic acid or thiocarboxylic acid (HS(O)CR) forming a water solublecomplex, such as M(S(O)CR)₂.xH₂O. Preferably, at least about 2equivalents of acid are added to ensure complete reaction with the metalcompound.

The solution, when pyrolyzed under N₂, leads to the metal sulfide.MCO₃+2HS(O)CR—H₂O→M(S(O)CR)₂.xH₂O+CO₂+H₂O

M(S(O)CR)₂.xH₂O+heat/N₂ MS+volatile by-products

The solution preferably has a phosphor precursor concentration that isunsaturated to avoid the formation of precipitates and preferablyincludes from about 1 to about 50 weight percent of the precursor.Preferably the solvent is aqueous-based for ease of operation, althoughother solvents, such as toluene, may be desirable for specificmaterials. The use of organic solvents can lead to undesirable carbonconcentration in the phosphor particles. The pH of the aqueous-basedsolutions can be adjusted to alter the solubility characteristics of theprecursor in the solution.

The maximum average stream temperature in the furnace when makingphosphor powders are typically in a range of from about 500° C. to about1800° C., depending upon the phosphor that is being produced.

While particles initially produced by the furnace have goodcrystallinity, it may be desirable to increase the crystallinity afterproduction. Thus, the powders can be heated for varying amounts of timeand in different environments to increase the crystallinity of thephosphor. Increased crystallinity will lead to increased brightness andefficiency of the phosphor. If such annealing steps are performed, theannealing temperature and time should be selected to minimize the amountof interparticle sintering that is often associated with annealing. Forexample, yttria-based phosphors annealed at 1400° C. under air for 58hours can advantageously increase in brightness intensity by 160 percentor more due to an increase in the crystallite size.

Further, the crystallinity of the phosphors can be increased by using afluxing agent, either in the precursor solution or in a post-formationannealing step. A fluxing agent is a reagent which improves thecrystallinity of the material when the reagent and the material areheated together as compared to heating the material to the sametemperature and for the same amount of time in the absence of thefluxing agent. The fluxing agent, for example alkali metal halides suchas NaCl or KCl, can be added to the precursor solution where it improvesthe crystallinity of the particles during their subsequent formation.Alternatively, the fluxing agent can be added to the phosphor powderbatches after they have been collected. Upon annealing, the fluxingagent improves the crystallinity of the phosphor powder, and thereforeimproves the brightness of the phosphor powder.

As is discussed above, it may be desirable to form phosphor particleswith coatings thereon to enhance the stability or other properties ofthe phosphor powders. The phosphor powders of the present invention canbe coated using several different methods. For example, a colloidalphosphor powder and a soluble, involatile molecular precursor to thecoating can be suspended in the droplets so that the coating formsaround the phosphor when passed through the heating zone of the furnace.Alternatively, a soluble precursor to both the phosphor powder and thecoating can be used in the precursor solution wherein the coatingprecursor is involatile (e.g. Al(NO₃)₃) or volatile (e.g., Sn(OAc)₄). Inanother method, a colloidal precursor and a soluble phosphor precursorcan be used to form a particulate colloidal coating on the phosphor.

In yet another coating method, a volatile coating precursor or precursorsolution is sprayed into the furnace after a point where the phosphorparticles have already been formed. The precursor reacts to form acoating on the phosphor particle surface. These coatings can begenerated by two different mechanisms. First, the precursor can vaporizeand diffuse to the hot particle surface and thermally react resulting inthe formation of a thin-film by chemical vapor deposition (CVD).Alternatively, the gaseous precursor can react in the gas phase formingsmall particles (e.g. less than about 5 nanometers) which can thendiffuse to the particle surface and sinter onto the surface forming acoating. This reaction mechanism is referred to as gas to particleconversion (GPC).

In addition, a volatile coating material, such as PbO, MoO₃ or V₂O₅, canbe introduced into the reactor such that the coating deposits on theparticle by condensation.

An additional heating zone, such as a second elongated tubular furnace,can be added after the main furnace, but before the quench system, tocoat the phosphor particles with the desired coating.

EXAMPLES

The following examples are provided to aid in understanding of thepresent invention, and are not intended to in any way limit the scope ofthe present invention.

Example 1

This example demonstrates preparation of multi-phase particles of eitherneodymium titanate or barium titanate with various metals.

A titanate precursor solution is prepared for each of barium titanateand neodymium titanate. The barium titanate precursor solution isprepared by dissolving barium nitrate in water and then, with rapidstirring, adding titanium tetraisopropoxide. A fine precipitate isformed. Sufficient nitric acid is added to completely dissolve theprecipitate. Precursor solutions of various metals are prepared bydissolving the metal salt in water. The neodymium titanate precursorsolution is prepared in the same way except using neodymium nitrate.

The titanate precursor solution and the metal precursor solution aremixed in various relative quantities to obtain the desired relativequantities of titanate and metal components in the final particles. Themixed solutions are aerosolized in an ultrasonic aerosol generator withtransducers operated at 1.6 MHz and the aerosol is sent to a furnacewhere droplets in the aerosol are pyrolized to form the desiredmulti-phase particles. Air or nitrogen is used as a carrier gas, withtests involving copper and nickel also including hydrogen in an amountof 2.8 volume percent of the carrier gas.

Results are summarized in Table 2.

Example 2

A variety of materials are made according to the process of the presentinvention, with some materials being made with and some being madewithout droplet classification prior to the furnace. Various singlephase and multi-phase (or composite) particles are made as well asseveral coated particles. Tables 3 through 8 tabulate various of thesematerials and conditions of manufacture.

TABLE 2 Metal Composite Precursor(s) Temperature ° C. Carrier Gas 75/25Pd/BaTiO₃ nitrate 1000 N₂ Ag:Pd/BaTiO₃ ⁽¹⁾ nitrate 600–1100 air 75/25nitrate 1000 air Ag:Pd/BaTiO₃ 75/25 Ni/BaTiO₃ nitrate 1200 N₂ + H₂ 75/25Ni/Ne₂TiO₇ nitrate 1200 N₂ + H₂ 75/25 Cu/BaTiO₃ nitrate 1200 N₂ + H₂75/25 Cu/BaTiO₇ nitrate 1200 N₂ + H₂ 50/50 Pt/BaTiO₃ chloroplatinic 1100air acid

TABLE 3 Phosphors Reactor Carrier Material Precursor⁽⁴⁾ Temp ° C. GasY₂O₃:Eu Yttrium nitrate, chloride or acetate 500– Air dopant andeuropium nitrate⁽¹⁾⁽²⁾ 1100 CaTiO₃ Titanium tetraisopropoxide and600–800 Air, calcium nitrate⁽¹⁾ N₂, O₂ CaTiO₃ “Tyzor”⁽³⁾ and calciumnitrateitanium 600–800 Air, tetraisoperoxide and calcium N₂, O₂nitrate⁽¹⁾ CaS Calcium carbonate and thioacetic 800– N₂ acid, variousdopants as metal 1100 salts⁽¹⁾ MgS Magnesium carbonate and thioacetic800– N₂ acid, various dopants as metal 1100 salts⁽¹⁾ SrS Strontiumcarbonate and thioacetic 800– N₂ acid, various dopants as metal 1100salts⁽¹⁾ BaS Barium carbonate and thioacetic acid, 800– N₂ variousdopants as metal salts⁽¹⁾ 1100 ZnS Zinc nitrate and thiourea, various800–950 N₂ dopants as metal salts⁽¹⁾ ZnS Zinc nitrate and thiourea,MnCl₂ as 950 N₂ dopant⁽¹⁾ Ca_(x)Sr_(1−x)S Metal carbonates or hydroxidesand 800– N₂ thioacetic acid, various dopants as 1100 metal salts⁽¹⁾Mg_(x)Sr_(1−x)S Metal carbonates or hydroxides and 800– N₂ thioaceticacid, various dopants as 1100 metal salts⁽¹⁾ ZnS Zn_(x)(OH)_(y)(CO3)_(z)particles in colloidal 800–950 N₂ suspension, various dopants as metalsalts, thioacetic acid ZnO:Zn⁽⁴⁾ Zinc nitrate⁽¹⁾ 700–900 N₂ + H₂ Mixture⁽¹⁾In aqueous solution ⁽²⁾Urea addition improves densification ofparticles ⁽³⁾Metal organic sold by DuPont ⁽⁴⁾Some Zn reduced to Znduring manufacture, the amount of reduction being controllable.

TABLE 4 Pure Metals Material Precursor Temperature ° C. Carrier Gas Pdnitrate 900–1500 N₂ Ag nitrate 900–1400 air Ni nitrate 700–1400 N₂ + H₂Cu nitrate 700–1400 N₂ + H₂ Pt chloroplatinic 900–1500 air acid(H₂PtCl₆.H₂O) Au chloride 500–1100 air

TABLE 5 Metal Alloys Material Precursors Temperature ° C. Carrier Gas70/30 Pd/Ag nitrates 900–1400 N₂ 70/30 Ag/Pd nitrates 900–1500 N₂ 50/50Ni/Cu nitrates 1100 N₂ + H₂ 50/50 Cu/Ni nitrates 1200 N₂ + H₂ 70/30Cu/Zn nitrates 1000 N₂ + H₂ 90/10 Cu/Sn nitrates 1000 N₂ + H₂ 50/50Pt/Pd chloroplatinic 1100 N₂ acid palladium nitrate

TABLE 6 Coated Particles Core Coating Coating Reactor Carrier MaterialPrecursor(s) Precursor(s) Method Temp ° C. Gas PbO iron sulfatePb(NO₃)₂in PVD  900 H_(2 + N) ₂ coating on in aqueous aqueous mixtureFe₃O₄ core solution solution Pb coating iron sulfate lead nitrate PVD 900 H₂ + N₂ on Fe₂O₄ in aqueous in aqueous mixture core solutionsolution PbO Ruthenium Pb(NO₃)₂ in PVD 1100 N₂ coating on nitrosylaqueous RuO₂ core nitrate in solution aqueous solution MgO Bismuth andMagnesium CVD  800 O₂ coating on ruthenium acetate in Bi₂Ru₂O_(7.3)nitrates in aqueous core aqueous solution solution SiO₂ Palladium SiCl₄CVD 1100– N₂ coating on nitrate in 1300 Pd core aqueous solution TiO₂Palladium TiCl₄ CVD 1100– N₂ coating on nitrate in 1300 Pd core aqueoussolution

TABLE 7 Composites Reactor Carrier Material Precursor(s) Temp^((c)) GasPbO/Fe₃O₄ Colloidal suspension of Fe₃O₄ 500–800 Air particles in aqueoussolution of Pb(NO₃)₂ Pd/SiO₂ ⁽¹⁾ 60 nm SiO₂ particles suspended in900–1100 N₂ aqueous solution of Pb(NO₃)₂ Pd/SiO₂ ⁽²⁾ 200 nm SiO₂particles suspended in 1100 N₂ aqueous solution of Pb(NO₃)₂ Pd/BaTiO₃Pd(NO₃)₁ Ba(NO₃)₂ and Ti(NO₃)₄ 1100 N₂ in aqueous solution Pd/TiO₂ ⁽⁴⁾Pd(NO₃)₂ and Ti(OiPr)₄ ⁽³⁾ in 1100 N₂ aqueous solution Pd/Al₂O₃ ⁽⁶⁾Pd(NO₃)₂ and Al(OsecBu)₂ ⁽⁵⁾ in 1100 N₂ aqueous solution Pd/TiO₂ ⁽⁷⁾PdNo₃ in aqueous solution slurried 1100 N₂ with 0.25 micronTiO_(2 particles) Ag/TiO₂ ⁽⁸⁾ Ag(NO₃)₂ aqueous solution with  900 N₂suspended 0.25 micron TiO₂ particles Pt/TiO₂ ⁽⁹⁾ K₂PtCl₄ aqueoussolution with 1100 N₂ suspended 0.25 micron TiO₂ particles Ag/TiO₂ ⁽¹⁰⁾AgNO3 aqueous solution with  900 N₂ colloidal TiO₂ particles Au/TiO₂⁽¹¹⁾ Colloidal Au and TiO₂ particles in  900 N₂ aqueous liquid.⁽¹⁾Morphology of particles changes from intimately mixed Pd/SiO₂ to SiO₂coating over Pd as reactor temperature is increased. ⁽²⁾Coating of Pd onSiO₂ particles. ⁽³⁾Titanium tetraisopropoxide. ⁽⁴⁾Metal dispersed onhigh surface area TiO₂ support. ⁽⁵⁾Al[OCH (CH₃)C₂H₅]₃. ⁽⁶⁾Metaldispersed on high surface area Al₂O_(3 support.) ⁽⁷⁾Pd coating on TiO₂particles. ⁽⁸⁾Ag coating on TiO₂ particles. ⁽⁹⁾Pt coating on TiO₂particles. ⁽¹⁰⁾TiO₂ coating on Ag particles. ⁽¹¹⁾TiO₂ coating on Auparticles.

While various specific embodiments of the process of the presentinvention and the apparatus of the present invention for preparingpowders are described in detail, it should be recognized that thefeatures described with respect to each embodiment may be combined, inany combination, with features described in any other embodiment, to theextent that the features are compatible. For example, any or all of theaerosol concentrator, aerosol classifier, particle cooler, particlecoater, particle modifier and other described process/apparatuscomponents may be incorporated into the apparatus and/or process of thepresent invention. Also, additional apparatus and/or process steps maybe incorporated to the extent they do not substantially interfere withoperation of the process of the present invention or the apparatususeful therefor.

Also, while various embodiments of the present invention have beendescribed in detail, it is apparent that modifications and adaptationsto those embodiments will occur to those skilled in the art. It is to beexpressly understood, however, that such modifications and adaptationsare within the scope of the present invention, as set forth in theclaims below. Further, it should be recognized that any feature of anyembodiment disclosed herein can be combined with any other feature ofany other embodiment in any combination.

1. A method of preparing a particulate product with multi-phaseparticles, the method comprising: generating droplets dispersed in aflowing gas stream, the droplets comprising a liquid and precursors fordifferent material phases of multi-phase particles, the precursors inthe droplets comprising a first precursor for a component of a firstmaterial phase of the multi-phase particles and a second precursor for acomponent of a second material phase of the multi-phase particles;forming the multi-phase particles each comprising the first materialphase as a disperse phase dispersed in a matrix comprising the secondmaterial phase, the forming comprising removal of liquid from thedroplets and chemical reaction of the first precursor at an elevatedtemperature in a reactor.
 2. The method of claim 1, wherein during theforming, the second precursor is not chemically reacted.
 3. The methodof claim 2, wherein during the generating, the first precursor and thesecond precursor are both dissolved in the liquid.
 4. The method ofclaim 1, wherein during the forming, a maximum average streamtemperature in the reactor is in a range of from 500° C. to 1500° C. 5.The method of claim 1, wherein during the forming, a maximum averagestream temperature the reactor is in a range of from 900° C. to 1300° C.6. The method of claim 1, wherein the reactor comprises a tubular hotwall furnace.
 7. The method of claim 6, wherein a maximum average streamtemperature within the furnace is attained in a final one of multipleheating sections of a heating zone of the furnace.
 8. The method ofclaim 1, wherein during the forming, residence time for flow through thereactor is in a range of from 0.2 second to 4 seconds.
 9. The method ofclaim 1, wherein during the forming, residence time for flow through thereactor is in a range of from 0.5 second to 2 seconds.
 10. The method ofclaim 1, wherein during the forming, flow through the reactor is at aReynolds number of from 500 to 10,000.
 11. The method of claim 1,wherein during the forming, flow through the reactor is at a Reynoldsnumber of from 1,000 to 5,000.
 12. The method of claim 1, wherein thereactor comprises a flame reactor.
 13. The method of claim 1, whereinthe reactor comprises a plasma reactor.
 14. The method of claim 1,comprising, after the forming, cooling the multi-phase particles whiledispersed in the flowing gas stream.
 15. The method of claim 14, whereinthe cooling comprises introducing a quench gas into the flowing gasstream to lower the temperature of the flowing gas stream.
 16. Themethod of claim 14, wherein residence time of the flowing gas streamfrom a maximum average stream temperature of at least 500° C. in thereactor to a temperature of lower than 200° C. during the cooling isshorter than 2 seconds.
 17. The method of claim 16, wherein residencetime of the flowing gas stream through the reactor and until reaching atemperature lower than 200° C. during the cooling is shorter than 5seconds.
 18. The method of claim 1, wherein the multi-phase particleshave a weight average particle size of larger than 3 microns.
 19. Themethod of claim 1, further comprising, after the forming,compositionally modifying or structurally modifying the multi-phaseparticles.
 20. The method of claim 1, wherein the generating comprisesproducing the droplets from a spray nozzle atomizer.
 21. The method ofclaim 1, wherein the generating comprises sweeping away with carrier gassaid droplets as said droplets are released from a reservoir of anultrasonically energized flowable medium, said flowable mediumcomprising the liquid and the precursors and said flowable medium beingultrasonically energized by a plurality of ultrasonic transducersunderlying said reservoir.
 22. The method of claim 21, wherein themulti-phase particles have a weight average size of from 1 micron to 5microns.